I've never used a beta reader, mainly because I never really thought about it. Although I suppose you could say my critique partner is my beta reader because she reads the entire manuscript pointing out what works, what doesn't, stilted dialogue, dropped plot threads and the like.
In the past 6 months or so, I've had the honor and priviledge to be a beta reader for two fellow romance authors. Both were great stories that I can't wait to see in print. After serving as one of their first readers, I can see the benefits of using beta readers. Not having any background or knowledge of the stories I was asked to read, I was able to come to the manuscripts pretty much as any reader would who purchased the books from a store. I was able to see things that maybe the author didn't realize were there or not there as the case may be. I was able to serve as a second set of eyes that wasn't biased by previous versions of the manuscript. I also felt like I was paying forward all the knowledge other writers shared with me when I first started writing.
It was a rewarding experience in both cases and has convinced me to use beta readers for my own works in the future. If you have the chance to be one, take it. You won't be disappointed.