Or are you forced to buy or rent a DVD to watch the deleted/additional scenes to find out what they're about?

Essentially, I'd like to read the description of the scene before buying the movie to see if it would be worthwhile? Are there any sites dedicated to this purpose?

Dedicated? Dunno, but some DVD reviewers will discuss it in their reviews.

Not dedicated, but among other things, The Internet Movie Database has sections for "DVD details" and "alternate versions" which you can respectively check whether deleted scenes are on the DVD, and what scenes have been changed for theatrical/home/overseas versions of the movie. I'm not sure how complete the site is in general, but there were good descriptions for the two movies that happened to pop into my head, Fight Club and Meet the Parents.

I second btribble. In the alternate versions section, usually one of the lines is "The 2003 DVD release contains 5 deleted scenes, including....."

I personally would rather watch them first than find out what they were before hand, but that's me.

Would you ever really make a purchasing decision based on what deleted scenes are included?

I think Blockbuster or netflix.com lets you check out films to see the deleted scenes before you actually purchase the disc. I may be wrong, though...

Only if it would be a repeat purchase. If I already have the DVD and they release a SE of the same movie, my interests generally lie in deleted/additional scenes and cast interviews as opposed to commentary or sound (since I don't have a "system" to speak of at the moment - just television speakers - oh, how 1993). IF the deleted scenes amount to nothing of interest - then I won't bother.

One example would be The Doors. I owned the VHS version, and didn't bother with the DVD because I thought the movie was just so-so. But because I was intrigued by all the deleted scenes, I picked it up for $9.99 at K-Mart. The scenes weren't that great (except for the one where Morrison spits in the cop's face).

If they're loaded with good stuff, then I'd rebuy.

If I didn't have the movie already on DVD, then I'd buy it regardless if it even had extra features.

DVDFile has a good database of cut footage and alternate versions.

I can just see a future thread:

"The Matrix Reloaded: Are the deleted scenes a good blind buy?"