I bought both the G7 and G15 and ended up returning the G15.
The G7 is a dream in terms of performance. It either equals or surpased all other mice out there, wired or wireless. It's very responsive. I love the ability to select the mouse tracking sensitivity on the fly. I do wish it had a few more buttons, but, I really don't miss them. My last wireless mouse was a Logitech MX1000. The MX1000 had good button layout and comfortable feel, but, it had this annoying delay when you move it after it's been stationary for a little bit (i.e., it takes too long to wake up).
The G7's one drawback is that it eats through a battery in about a day of decent usuage (call it 6 to 8 hours tops). Draining these batteries dry between charging means that their overall life won't last that long, even though you get two batteries to start with. I'm guessing they will last about a year before you start to notice serious capacity degradation.
In my case, my G7 had a defective USB battery charger --- it shuts off charging prematurely and so I ended up with no battery power at times. I got in touch with Logitech tech support who said that they'll ship me a replacement charger. It's been four weeks and the darn thing hasn't arrived yet. The tech support guy thought it should arrive in two. I'm using a G5 wired mouse to hold me over until the G7 is back in business.
The G15 was a complete disappointment for me. First the positives: 1) it's the nicest looking self illuminated keyboard I've seen; 2) the little LCD screen can provide interesting realtime info such as how much CPU power is being gobbled up; 3) the switch to disable windows keys so that you don't accidentally press them is a nice touch. The negatives: 1) the light that shows which macro key set is selected is really bright and you can't shut it off --- I found it distracting while gaming; 2) the volume control knob has no tactile knurling on it so it's too slippery to twirl for quick/fine volume adjustment; 3) very few games support the LCD and the ones that do (e.g., Civ4) illustrate how the little LCD doesn't manage information nicely --- hard to read lots of lines of information and the ability to scroll through it using the special LCD related keys are a pain in part because those keys don't light up and so are hard to find in the dark. Finally the indifferences: 1) all those macro keys for me remained unused; 2) I'm not too crazy about the keyboard feel (I much prefer the OEM Logitech Ultra Slim keyboard feel); 3) the keyboard is too wide because of all the macro keys along the left edge.
So in brief, this keyboard is about the macro keys and the LCD. If you don't need the macro keys --- or can't get used to quickly pressing the right macro key for your particular game --- or don't need the limited use LCD, the G15 just doesn't bring enough to be worth the price. In the end, I went back to more traditional keyboards that has the few extra buttons I do use a lot (e.g., volume control, hot key for calculator, etc.).
I do worry about Logitech tech support, but, to be fair, I'm the only person I know with G7 problems --- my six other friends' G7s are just fine.