I don't have a lot of experience with them, but I'll share my thoughts on the 2 I have used.
I have a Square swiper that I use with my Droid X. I don't use it very often, but it seems to work well. Square's pricing is really good if you are doing low volume or low value transactions.
I just had a client that ordered one of Intuit's GoPayment swipers. It apparently doesn't work with every Android phone; it doesn't work with a Droid X or Droid X2. It did work alright with a iPhone 4S. I didn't think the swiper itself was as good as Square's.