I have 3 G2s including the 7'7" 4wt. I use a Galvan Standard 2.75 on it and it balances fine. I'd say if you have a reel that is 4.5-5.0 ounces or lighter on that particular length rod, it will feel fine. If you have a longer rod model, then you can go even a half ounce heavier due to the additional swing weight the added rod length adds out at the tip.
As far as opinions of the series: I love them. I use them for all my primary trout rods up to the 9'0" 6wt. Casting is super smooth and relaxed, though you can add some punch when needed. If you are a proficient caster, you'll do fine with the rod in all situations including heavy wind. I like the medium action for protectiing tippets and otherwise helping me land fish vs. popping them off from a too-stiff rod.
I have the 8'8" 4 weight and balance it with a Ross Evolution 1.5. I've also mounted a Ross Cimarron (standard arbor, not CLA) 2 on it, but felt that it moved the overall balance more toward the hand--not necessarily a bad thing, but I like it better with the Evo--the rod's balance felt more "neutral" as opposed to feeling butt or tip heavy. In that regard, I agree with Lugan on the reel weight range and would say that these rods (in my opinion), especially the shorter ones, would appreciate a heavily ported reel. Ross Evolutions, Waterworks ULA's, Sage Click series, Lamson Litespeeds, etc. would be perfect matches.
In terms of rod usage, I've been using the rod primarily for large trout (to me, that's anything bigger than 18") over the past few years and despite its medium flex profile and small fly, delicate presentation reputation, I'm still able to throw larger, heavier flies (i.e., conehead wooly buggers, medium leeches, etc.) with a good degree of accuracy. It's also been used to nymph and, due to its lightness, is quite a pleasure to use for this purpose. All in all, I've found it to be very versatile, able to handle most tasks you might need to throw at it. And they're pretty rods, too.