In addtion to feeling comfortable, make sure you check out the MOI rating. The higher this rating, the more the putter face will resist twisting on off "sweet spot" hits. First thing I do when looking at putters is to hit balls off the toe, center, and heel of the putter face. When you find a putter that sends the ball reasonably straight off all three of those putter face areas, you are going to make more putts with that putter. That is more putts when compared to a putter that is more prone to twisting on off sweet spot hits. BTW, the putter's sweet spot in not always in the center of the club face.
http://www.golf.com/golf/instruction/article/0,28136,1623623,00.html
Iam also agreeable that a putter should be properly fitted to the golfer by a qualified club fitting guru. Loft angle, lie angle, and shaft length can all be adjusted to help the golfer make more putts, based on the individual's putting stroke.
It's all about making more putts.....FHs