I have always enjoyed Logitech mice, but hated how dog fur would get into the sensor area and create all sorts of tracking issues. This one maintains the areas Logitech is always strong, and really improves itself in some areas that no cordless mice are usually good at (like Pug fur resistance).
First the battery life is incredible. I've only been using this mouse for about three weeks now, but in that time, my other cordless mice usually need to be recharged. This one is still full. I have no reason to doubt, with its motion activated control that the batteries will last a very long time.
The ultra compact wireless receiver is not a brand new feature, but it continues to be innovative. Compared to the wireless receivers on most of my cordless mice, this one works great and is extremely compact and unobtrusive.
The comfort, tracking, and accuracy of this mouse is as good, if not better than my other laser mice. I don't know what type of tracking technology is behind the sealed sensor on this, but it works great on most surfaces, and performs better than any mouse I've ever tried on fabrics, couches, and bedding. It tracks as well as my Logitech MX1100 on the surfaces that one will work on, which is not nearly as many as the M515.
Even if your mouse never leaves your desk, you should consider the M515 if you have pets. The fur from our two Pugs constantly manages to get into the sensor on my other mice. With the sealed sensor on the M515, that problem goes away immediately.
The size and features of this mouse are perfect for me. No better or worse than the MX1100 or my other mice.
I recommend that you try this mouse if you use your computer on couches, beds, or other non-ideal locations.
---- Update June 2011 ----
After six months of use, I am still very happy with this mouse. It works great, stays clean, and isn't affected by dirt, dog fur, or any of the other things that plague my other mice.