I use it. Very cool site, and I've 'met' some great people from all over the globe. However, I am also dumber than a bag of hammers, so I make a ton of mistakes on other people's songs. Thankfully, no one has had me barred from kompoz yet.

I've taken bed tracks from people with no click, and no click I try on my end seems to line up. I've taken tracks that the person swears is in E but I get F# no matter how hard I try to get E out of it. I am starting to think it's all in how they recorded and no one tells me their sample rate. I may be wrong, but if I use 44.1KHz and they use 48KHz, as an example, maybe that throws a monkey wrench into everything?

Regardless, the songs that did work out for a click and key; I had a blast, and I think the final 'product' turned out great.
In a nutshell, you join for free. You can pay, but all you get as an 'extra' is more choices for upload/download formats, and someone somewhere at kompoz (I have no idea who, how, or where) hawks your final product for placement. I've been doing the freebie for a while, and as I say, there's some great songs up there, and you can add whatever instrumentation you feel like doing. Or sometimes I just remix someone's song. You're only limited by your 'studio' and your imagination. And it's not all high-end studio recordings, either. I've heard some lo-fi recordings, and some with distorting bass and some with out of tune guitars! It's still all good, and a great learning experience. I say go for it.