Ok, here's a link to <A HREF="http://www6.tomshardware.com/storage/01q2/010404/index.html" target="_new">Tom's Review</A> of the Seagate Cheetah x15. And don't even think about asking for other website's reviews
To sum it up, it's quiet and doesn't vibrate much, but it's expensive, not as high a performance gain as you might expect for a 15k drive (due to lower density). Now, it just depends on what you're using it for. It has huge access times...I mean, really good access times, but its transfer rates aren't as good. If you're using it in a server with small files, it'll do great. But, if you're going to be doing large audio or video files on it, you'd be better off with a good 10k drive.
At $410 for an 18gig drive, it's about the most expensive out there. Tom doesn't say whether or not is has a built-in controller, but it looks like it doesn't. In which case, get a good Adaptec Ultra-160 controller, as stated above.
Ok, two last things...firstly, it does get quite hot (I remembered wrong, apologies to mpjesse). So, a case fan or two is highly reccomended, although that'll probably still do the trick. The best place I've seen for cooling products is <A HREF="http://www.3dcool.com" target="_new">3Dcool.com</A>.
Secondly, I just thought it was interesting that Seagate sent a refurbished drive to Tom for the review. He doesn't say that, but it has a big "Factory repaired" sticker on it. Hmm...
Apple? Macintosh? What are these strange words you speak?