TakeNGive: I haven't tried this myself; would be interested in others' opinions.
mAsterdam: I like the look, the feel and the power of the program. I would like it even better if it had more extensive editing support like the game-tree display in CGoban 2. I am curious about the feature wishlists of the other deshi.
Aaron Cass: I like gGo. In particular, I like the way it deals with the game tree better than Jago, which I used to use. For example, suppose you are reviewing a game and are currently at a node A which has child nodes B and C, and you've just explored B and now want to explore C. In gGo, while viewing node A, you simply click the appropriate intersection for C. With Jago , on the other hand, you first step the game forward to node B and then indicate that you want to view alternative C. Basically, Jago seems to view different paths as alternatives to the current node, while gGo seems to view them as alternative children. gGo's approach matches my mental model better. YMMV.
Blake: gGo is a good program. However, it has one important flaw: it is a memory hog. As it is written in Java, it tends towards this to begin with; gGo, however, is somewhat worse in this aspect than either CGoban2 or Jago; when I start it up--without opening a board or anything--gGo can take more memory than Netscape 7.02 with several tabs open. Until this problem is addressed, I'm afraid gGo remains impractical for lower end machines (like mine).
holosys: I use gGo to play on IGS, play against GNUGo and read IGS PandaNet Magazine game files which are in .UGI format. For more general SGF markup I prefer the KGS client. gGo is definitely worth grabbing, it works really well and the graphics, board and stones are really nice (are they based on photographs?). Make sure you follow the installation instructions and are using the latest version of Sun's JRE (see http://holosys.co.uk/beginnersroom/ for a handy JRE link) and of course the latest version of gGo itself (see current version above), the one on the GNU Go site is out of date).
evand: the link on the GNU Go site is to the last Free version; this is very intentional, as GNU.org has a policy of not linking to non-Free programs.
ZeroKun: Oh man, i just upgraded to 4.2, which has a great new theme Liquid. Anyway only problem i have with it is it takes a bit more time to load than other clients, after that its perfect.
 At least in the version of Jago I was using. Please correct if this has changed in more current versions.