I've got the MX510 also and I would never go back to any of the others I've tried. It didn't feel right when I first got it, but after a couple of days it became second nature. Honestly for you though, you have narrowed it down to a couple which is good, but the best advice I can give you is to try the Intellimouse out before you decide. Everyone has different preferences when it comes to these things (see all posts above), and no one person is going to have the same preferences as you. Hopefully you have a friend that has the Intellimouse you are interested in so you can try it out, otherwise you can always go to the store and give it a semi-shot there. Good luck, and make the choice on your own!