They're dark speakers, that handle huge low end with no problem. They also take way more wattage than your average Celestion, which means almost no speaker breakup.
The Swampies sound huge, my 2-12 with them can easily match a 4-12, IMO. Great at taming the harsh highs of certain amps too.
The only complaint I've ever heard was they didn't record well for one guy on HC.