I must admit that I am biased to this series. I watched, and loved the UK version first, and as might be seen in general discussions, the second version you see rarely live up the the expectations raised by the first, since they are very different. Both versions pop up in various places on this site, and but the UK version has no page of it own, so I think a review if both could go here.
The original is much more narrow than the "sequel", focusing on the core cast and handling one or possibly two conflicts at the time. Theme-wise, it keeps its focus on the gay men, so the lesbians are supporting characters at best, and doesn't cover a fraction of the issues discussed in the American version. This can probably be at least partially blamed on the shortness of it (only 10 episodes total), though it isn't a watertight justification. But it's good in so many other ways for example, in spite of being fairly dramatic, there is a positive tone to the series and a lot of humor. It does bring up different themes, and often in a quotable way, but the series doesn't take itself too seriously. The casting and setting gives a natural, realistic feel, and the characters are well rounded and treated respectfully.
As for the American version, you really get a chance to get attached to the characters in its long run. There is lots of character development, which is really pleasing to see. It also cover a lot of ground theme-wise, everything from AIDS, to drug addiction, to cancer with plotlines that pretty much erases the risk of getting bored with it. The lesbian couple get a much bigger role too, but a mild version of the Smurfette Principle is still active here. For someone who started out with UK, the casting and charactarisation is a bit jarring. Every guy could be an underwear model, and after a while you notice that some nuances seem to be missing in their behaviour: Jerkwith A Heart Of Gold
aside, Brian is unpleasant to everyone all the time, Debbie's life circles around being the mother in the gay community, and Michael is shown as childish and obsessed with Brian. One can't quite shake the feeling of a love story with Brian as the dashing hero and Michael as the rival, especially with the at times gratuitous sex scenes and agressive defense thereof. Deus Angst Machina
seems to happen a lot too, resulting in everyone angsting all the time.