I went through their process and got lots of practice interviewing. Even though I didn't take any of the offers I got through them I was able to use one to negotiate a much higher salary at the job I did take. Their recruiters and interviewers treated me respectfully and professionally.
So I'd recommend it, especially if you're very good and/or smart but not an attractive candidate by conventional standards. If you have 5 years at Microsoft or something then you probably don't need them, since you can interview wherever you want at that point.