I have a Logitech G5, and I think the sensitivity button can be a bit annoying... I wish there were a button on the bottom of the mouse (recessed in) to lock the sensitivity. I've found myself wondering why my mouse was so sluggish only to realize that I accidentally hit the lower sensitivity button.
The weight is actually a nice feature on the mouse, although it may seem really ODD at first. Especially since you mentioned the MX1000, you're used to cordless mice. I had a MX700 before I got the G5 and I was also used to cordless mice. The thing about cordless mice is... they weigh significantly more than standard corded mice. So, to keep your hand motions the same and avoid the awkward feeling of a lighter mouse, you can simply put weights into your G5. I think I filled 5/9 of the weight spots with 4 grays and 1 black (I believe gray = 5g and black = 10g). It's also kind of amusing to launch the weight cartridge sometimes if you're bored

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I don't use the mouse's software either. There's really no reason to, since the mouse only has 1 extra button anyway and most if not all software already supports the back mouse button (MB4). I've also always disliked Logitech software, so I tend not to install it.
Overall, I think it's a great mouse and looks nifty too

. To rate it, I'd say 4.5/5 or so. It's a little expensive for a mouse and those sensitivity buttons can sometimes be a pain. Personally, I use level 2 sensitivity by the way. Level 1 is too sluggish and level 3 is waaaaay too fast.