I didn't think Sonic Boom was very good, some people really liked it though. It does come with a disc of re-recorded old songs and a short dvd, so there's at least a lot of bang for the buck. 

Their first (self titled) album actually has a lot of what I consider their classics on it and was one of the ones I saw for $5.

You know what, that's probably where my disconnect comes from on this.  The only reason I initially got an ipod was because it gave me the ability to travel with my entire music collection.  So if I'm in the mood for something, but not a specific album I'll just put that artists discography (what I have of it anyway) on shuffle.

All that I don't like about them is tossed aside if they have cool bonus content with them.  I got the Faith No More one because it came with a disc of bonus tracks plus a couple b-sides I hadn't gottten my hands on before.  The dvd that comes with the Stone Temple Pilots greatest hits is more comprehensive than most dvds band release just to stand on their own. 

That said, plenty of the illegal downloading I've done in my time was to get a few tracks that I couldn't get without buying an entire album of songs I already had or didn't want.

edit:

Haha. Sympathy For The Devil is one of those songs that I aquired outside the law, but to be fair, I did pay for the cassette single back in the day.

To me if you're a die hard of a band, and there isn't much or any new material on it, you shouldn't bother. 

Knowing how much you like the new Soundgarden album I'm going to guess that even if Telephantasm is great all it's going to do is whet your appetite more and you'll want to pick up the actual albums to get more tracks, then you'll just end up with multiple copies of the same songs.

Most of the greatest hits albums I have are from bands I don't plan on ever picking up many (or any) more albums from.  I'm glad they exist because of that, but to me they're not for a band you love, they're for bands you kind of like, or want to check out.