to clarify my above....I might not rush out and buy a seaclone, but if you already have one then it should do a passable job on a 55. I got one with a used tank (person was getting out of hobby type thing) and it has done just fine on tanks in the 55-75 gallon range. They don't always give you the "turkish grind coffee" skimmate, but you shouldn't have any problem getting "tea" and changing the cup a bit more. Still pulls out the fug...just not as nicely as others.
If you are buying a new skimmer for a larger tank then ASM, coralife, euroreef are all good models. FYI the coralife goes both in sump or hang on the back.
PS I've seen better deals on the ASM G3's before...can't find the site but a shot on google and some clicking around should get you a better price. Also keep an eye out on the classifieds section of this site...frequently people are selling/trading/etc.
Bottom line: If you have one already = better than nothing, but if you are spending there are many better options. Perfect for a secondary/extra/etc tank.