I just got sent a brand new MX1000, because my MX700 charger gave out it me. The 700 still works fine, but I was switching between two pairs of AA NiCads and a wall charger and figured Logitech could hook me up. The thing I noticed immediately is that ths MX1000 seems ... made more cheaply. The thumb buttons (3, the "application switch" button was shifter from under the scroll whell to between FORWARD and BACK) seem a bit too big and hollow or wiggly. There's just more plastic all around, which isn't necessarily a good thing. The thumbrest-deal takes some getting used to.
The scroll wheel's easier to turn now, with less defined notches, and it can also do left-to-right now.
The on-mouse battery indicator is nice.
The software, however, is blah. Mouseware won't touch the thing, and so you kinda have to use this new SetPoint. Takes up more memory than em_exec, which I'm not liking at all. Also, the FORWARD and BACK don't work by default in my Firefox 1.0 preview release - I had to assign the buttons ALT-LEFT and ALT-RIGHT keystrokes. The back button is also a bit further towards you on the mouse now, meaning you've got to bend your thumb more to reach it.
The laser makes a big difference where it doesn't really matter. I can use my mouse from on my bed's comforter now, hooray. Most people are already using a suitable-enough surface, though, I'm sure, so that probably won't sway any decisions.
The mouse is much lighter than a battery-loaded 700, which is also taking some adjusting.
Figured that could be helpful, since I think the 700 and 510 are essentially the same thing, just with different wire situations.