I've had three, and every time they've lasted me at least a year, if not more.
Lost the first one
Second one was broken by user error
Third one is still running strong.
Battery life is superb. Two AAs will last you at least six months, a year and a half at the most. It's stupidly good.
Not sure how many extra buttons you need, though. M510's got two thumb, but that's about it.