I used to think there was no replacement for Waves SSL -- nothing else gave me such snap and punch, particularly on drums -- but then I started experimenting and found I could pretty much duplicate that sound with other plugs. Waves' own RS56 EQ was a part of the recipe for the snare, for instance, as was the judicious use of transient designers. Now I no longer think it's a must-have, but I wouldn't say it's bad. I still think it's one of Waves' best plugins. But there are better values out there nowadays.
For UAD users, that $300 would be much better spent on the API channel strip, IMHO.
Cheers,
Eddie