The seats were ineffective above 80K feet on ascent due to the expansion of the SRB plumes - if you ejected after that, you would pass through the plumes making a bad day worse.  A call was made at that point "[Negative Seats][1]". [Note that the reference cites a thermal limit on the faceplate - I remember it was the plumes - take your pick.] 

There was a "speed limit" on ejection during entry above which the crewmember would be going too fast to thermally survive the entry.  Sadly I don't remember what that point was.

So, the ejection seats were usable only for a short time during ascent, and a short time during entry.  Worth it?  At the time it was thought yes, due to the fact that no un-crewed test flights were made.

FWIW, here's a picture from the STS-3 training era in the Shuttle Mission Simulator showing what the cockpit looked like then with the ejection seats with the large "rails" that connected the deck to the overhead.  Jack Lousma in the commander's seat and Gordon Fullerton is in the pilot's seat; I believe the lady is Mrs. Fullerton.

[![enter image description here][2]][2]


  [1]: https://blogs.nasa.gov/waynehalesblog/tag/ejection-seats/
  [2]: https://i.sstatic.net/UBuny.jpg