I've been with the XT for a while now (taken it through two firmware updates) and I've still mixed feelings about this drive- so can well understand your apprehension.
In terms of vibration and sound I can honestly say this isn't an issue (for me anyway- zero vibration and the sound isn't a problem- although it is noticeable as the drive never spins down with firmware SD25). Of course, this does make it snappier and working with files that need fast reading/writing (like photos in Aperture) is amazingly better. There is also a significant difference with boot times and launching large programs (my 200gb+ iTunes library is 'bop bop pop' there) which is also nice but not as critical.
The reason for my reservations is that the battery has taken a significant hit- depending on what I'm doing I can expect 4-6 hours out of it which has really had an impact on how I use my (2010 13") mbp. Also, there does seem to be a quite a difference in how this drive works for people- its not consistent and perhaps that should be kept in mind, although so too my feelings on it: I've been absolutely LOVING it recently, although can't say that was the case when I wasn't using its full potential before!)
From what I read on the web (and Seagate's forums are a good source of developments- and whining complaint!) there is a firmware update on the way- although the new drives ship with SD26 which I can't comment on as it isn't available to download, my understanding is that Seagate have stopped releasing frequent updates and are working on a proper 'fix' that should sort this drive out once and for all.
So, tl/dr: I like this drive; although its not perfect it has really made doing the stuff that I do a LOT better, and for the price I don't complain that it can't do it for 8+ hours away from the plug. If you do data intensive stuff and aren't away from the mains the whole day it could be perfect for you. Hope this helps you some!
