Received opinion would have you believe that by the end, "E.R." was not the show it once was, but over its fifteen year run, this show seldom failed to be anything less than top notch quality TV drama.
This, the final season, is truly a return to the series at its very best and it goes out on an incredible high note that makes you realize how much it is going to be missed. At its best "E.R." could always move its audience emotionally and stories in this season are amongst the best at achieving that. The final episode is a cracker, which manages to reflect many of the themes and moments of the pilot showing the cyclical nature of the world that Michael Crichton and his team created. On the strength of these shows you realize that the show could have gone on and on, but perhaps it is better to let it go now whilst it was still producing episodes of such quality.
Over the course of this season, pretty much every main character that ever featured in the show manages to make an appearance in one way or another, and the main cast are given opportunities to shine, grow and develop and even previously less than well served characters grow and mature as the show and their characters achieve a rare opportunity for proper closure. Even new, supposedly "unlikeable" characters become whole rounded human beings over the course of this run and some of the old favourites finally get the departures to new lives that their characters deserve.
If you only ever buy one season of "E.R.", I'd still recommend the first, but if you want two, then you could do far, far worse than to get this one.