|Test knits are the best win-win that I know.|
I'll start by simply saying this ~ being a test knitter isn't hard at all. It's actually a lot of fun. There are designers out there who need a hand in making sure that all of their sizing work is correct - that is where test knitters come in handy. We also check for errors in the patterns, ease of understanding, etc. It's basically a sneak preview of some wonderful items and helping to put the final touches on them before the rest of the world gets a shot at them.
The only real responsibility that a test knitter has is that you are honest with your time. The designers put in a time frame for when they need it done and they list how many testers for each size. They outline roughly how many hours they think a person will need in order to finish their project so you can see if the time required meshes with your time available. For example, the sweater that I am working on has a time allocation of 30 - 40 hours and a deadline of 8/30. I have spoken to the designer and let her know that I am not the fastest knitter but I really wanted to participate. She said she isn't the fastest either and had no issue with me testing the size that I requested. So basically, if you can't dedicate the time then don't make the offer because it isn't fair to the designers and it will earn you a bad reputation around the testing pool area.
I became a test knitter through the groups on Ravelry. I participate in two groups and usually only offer my services for one project at a time. I have a friend who can knit like the wind and handles multiple projects - there are limits set forth by the groups but they are generous. The end reward for the tester is a pretty new item and free pattern - for the designers, it is help with the final details and validation that their pattern is perfect and ready for the world.
So there is a brief overview of my life as a test knitter. It is fun and I highly recommend it to anyone who knits. As a tester and designer, the value of test knitting is priceless.