If the search for life on Mars is a particular concern, I think putting boots on the ground would be devastating (or bring smashing success, depending on your perspective). When we send robotic probes to Mars, we do our best to send them sterile so as to not contaminate our search. If we send human beings, which contain far more microbial cells than human cells, there is no chance of avoiding contamination. We would wear space suits while out on the Marian surface, but we can't put those suits on without smudging millions of bacteria all over them. Maybe before we step out, we bathe the outside of the suit with bleach and radiation, but the chances are approximately zero that we will always be able to guarantee a clean suit.
Essentially, if we go there ourselves with all of our dirty biology, I can guarantee you that we will find life. Even if Mars is barren today, once we arrive, life will find a vector to escape from its human host to the Martian soil. Now, in the long term, I think that's a good thing. I don't think a barren planet is as valuable to the Universe as one where life is growing and evolving, but of course, I'm very biased, being alive. But I do think that if we want a clear-cut answer to whether life already exists on Mars, we need to conclude our search before we send living explorers. It's certainly possible even after our arrival to find fossil evidence that predates our arrival or to prove distinction of a Martian genetic strain, but without a doubt, the core question of life on Mars becomes more complicated and difficult once we contaminate the site, and it casts doubt on anything that we might hope to find. And I can just imagine the articles that our descendants may one day write about how our generation hastily flubbed the search for life on Mars.
Edit (yes, everything from here down): Mark Adler has pointed out in comments below that I have not made clear how contamination would be a problem. Thank you Mark. You're right. I see 4 varieties of life that we might discover on Mars which, by their nature, would each have vastly different degrees of similarity to Earth-life: 1) Completely independent Martian evolution. Probably not even DNA-based. 2) Seeded by galactic or intergalactic panspermia. If this turns out to be a real thing, Earth would have been similarly seeded. 3) Branched from Earth during the last couple of billion years (Think impact ejection dispersal; not Cambrian spaceships.) 4) Brought by humans.
Now, depending on your philosophy, you may be tempted to scoff at one or two of the possibilities I have mentioned. But I ask you to consider the fact that nothing has been or can be proven yet about the feasibility of these proposed sources for life, or even which is most likely. So far we have no scientifically admissible and conclusive evidence that life exists outside of our little spheroid, which is precisely why this question is so important on Mars and anywhere else we explore. So if you scoff, it is a philosophical scoff, not a scientific one. (Please correct me if you can prove anything contrary.) I'm not as worried about missing a variety 1 discovery through contamination, but missing varieties 2 and 3 would be every bit as tragic a loss, and we just don't know how easy it might be to miss. Imagine actually finding them, but then later in peer review not being able to prove that they didn't come from Earth through contamination, because they are so similar to Earth life.
In the event of contamination, whatever little Earth bug manages to thrive in a place like Mars will likely be pretty unique. The creatures that we have found in the least Earth-like places on Earth, places like the Yellowstone caldera, are not your standard garden variety e coli. They are still recognizable as Earth life, but remember, we need to be able to distinguish them from Martian life variety 3.
I would like to hope that our Earth muck would not spread nearly as successfully as any existing Martian bugs growing in their home habitat. Our bacteria ought to need millions of years to adapt as well as the Martian native life. And if some Terran extremophile did manage to start spreading relatively quickly through the subsurface of Mars, we could take that as evidence that there was nothing native to Mars before our arrival, because clearly if any lifeform can spread like this, why wasn't Martian life all over the place and easy to find before this contamination event? OR, it may be that our biota is simply so wealthy with genetic variety due to the ideal planet we developed on that it is better armed to crush the sparse native Martian population under it's heel, and we will never even know it was there. If we could have found it, yes, we might have been able to distinguish it from Earth life, but it will be exterminated in short order by our invading bacteria in this scenario. If something like this did occur, I tend to favor the probability of the former explanation to the latter, but again, no scientific experiment can actually prove that. We have only philosophical conjecture.
The worst case scenario is that there is life on Mars and the life we are looking for is of variety 3 - It's originally from Earth and therefore somewhat similar to potential sources of contamination. Then contamination occurs and doesn't look like anything we've seen before, because it's a rare little beastie. And then worse, it ends up spreading much more successfully than the uncommon Martian life that is there already. Finding native life at this point would be worse than looking for fireflies while the forest is on fire.
But how, you may ask, could the contaminant species possibly be more successful than the Martian species that have the advantage of eons in the Martian environment, but are otherwise similar (from Earth)? I would expect that it would not be, from my perspective in an absolute vacuum of actual scientifically tested knowledge on the subject. However it could be that the advantage of eons on Mars is dwarfed by the advantage of the species that hitched a ride to Mars not on the molten embers of an exploded planetary surface at the point of some massive impact event which then cooled to temperatures that made Mars seem tropical by comparison and then drifted, degrading gradually, for many millions of years before then barely surviving another hot impact event on the surface of its new home, but instead inside a nice temperate wet human with a spaceship that provides such an incredibly safe and quick ride that even the over-sized eukaryote manages to survive and then do SCIENCE after his/her arrival. Maybe.
Now, you may notice that I have claimed that there is little we can prove, that we can only guess at what might be out there and what could happen during contamination, then I painted a relatively specific picture of what I would hope to avoid. So recognize that I'm not warning that this great bad thing will happen if we send people too early. I'm pointing out the risk of what could happen. And I see no good way to accurately measure that risk.
This article was published at phys.org today (Oct 21, 2016). The final paragraph corroborates the primary point of my answer, as these experts in the field express concern about the spoiling of the pristine Martian environment even by remote landers.
The concerns I raised here would also be significantly validated if what they found with the LR experiments turns out to truly be biological: They describe the hypothesized microbe as fragile (very sensitive to temperature fluctuation at least). Thus, if it does exist, it may be easy to inadvertently exterminate.