I'd say get the best, most comfortable mouse you can get your hands on. It doesn't have to be the G5, in fact, some people complain that they're too big. But if you're used to your MX510, the G5 is a good choice.
As for the G15, I
own one. But I wouldn't say I recommend it. Its just way too expensive to actually be of any use. I almost never use the LCD screen. I occasionally use it to tell the time (during full screen games) or to tell how far through a boring movie I am (without moving the mouse and disturb the boring show).
In fact, I knew beforehand that it would be this way. Before this I've always been doing pretty well on a normal keyboard with NO bells or whistles. But then, I saw this keyboard on a secondhand forum for really cheap.... why not?
I wouldn't say it cannot replace a normal keyboard, it doesn't present with any problems (the blue light can be switched off too!) except one: the ctrl key is no longer the bottom leftmost key anymore, and may be a slight adjusting problem. I keep reaching for the corner key and hitting one of the mod keys.
It still looks cool though, a great thing to lug around to LAN parties for people to gawk at. Works just as well as a 30" display, but much cheaper