I have one, and it's probably one of my favourite mice so far.
There are a few minor issues, though;
Ambidextrous buttons, so I tend to accidentally press the right side buttons by accident, but this isn't a problem if they're unbound
Laser sensor, so slight hardware accel, but really not noticeable in practice, at least not for me, and I do use it for precision FPS games
What I really like about it is the software is nice and tidy, it looks really good and while it'd be awesome if the silver part was actual metal, it's still a really nice surface since it's fairly grippy, but it doesn't wear and go ugly. Unlike the rubber coating you find on most mice which tend to start showing wear after just a days use.
Also; it's not really "pro grade", most pros generally don't use the top-range mice as they're expensive, use laser sensors which offer no real advantage, only drawbacks, and they often need software installed to get the most out of them. Pros and other serious precision gamers often prefer mice that are easy to use, if they can avoid any software at all that's a huge plus (physical dpi and polling switches underneath the mouse as on the Abyssus) since that means getting set up at LANs and tournaments is fairly easy, optical sensors as those are generally perform better than laser without hw accel, tracking speed etc, and it's also a plus if they're cheap since that means it's not as annoying if you have to replace them(until you get on a team with a sponsor, if you happen to use sponsor mice then you just get new ones for free I assume)
Still, despite not being "pro grade", that's not really all that relevant outside the competitive scene, and I've found the Sensei to be one of the more enjoyable mice to use so far of the ones I've tried.
