Lescott has been excellent over the last few months. We've kept a lot of clean sheets in that time, and he has been a big part of that. It's only right that he gets some of the credit.
As against that, while he is generally reliable, he has vast, gaping weaknesses in his game that (given his age, the coaching he's had to date and his physical and technical characteristics) will likely never be improved, or at least not significantly.
I think there are three significant problem areas in his performances for City. These are:
1) Technical ability. His all round technical ability - short/long passing, dribbling, control - is the worst of all the defenders currently playing regularly for the top five teams.
2) Defensive ability against short/quick/agile players. Lescott can handle large, physical, immobile centre forwards and aerial battles adequately. He is vulnerable against highly technical players or players who like to get in behind the defence. There are lots of these players in the Champions League...
3) Concentration/mistakes. Stoke's only chance on Saturday is an example. That was a high ball that Lescott should have dealt with, but he isn't totally reliable in these situations. Given his other weaknesses he really, really needs to be.
A really excellent footballing centre half would make an enormous difference to our side. That doesn't mean we should replace Lescott, though, as finding such a player is rare and even if we did, spending on one might not be the best use of our resources. A midfielder might be a more important purchase, and you do have to weigh up the disadvantages and the risk of breaking up a partnership that has started to bear some fruit.
I wouldn't replace him this summer, on balance, but definitely a top drawer centre half is an important upgrade that the management need to find in the medium term. Boyata, perhaps...