I don't currently use one as I haven't needed one thus far -- parameters have been great, current fish thriving -- but I want to eventually move my eels into my main tank until I get my bigger tank, as my other fish are far too big to be eaten and being that they are carnivorous, I'm going to need one. My filtration is currently a Marineland C530 (bio, carbon, mechanical) and 2 Cascade 300 HOB (strictly bio), plus tons of live rock and live sand. I am looking to get an external skimmer, preferably HOB as that's all that space permits unless I find someplace else to keep my supplies and clear out inside of my stand next to my canister filter. The ones I'm finding at my favorite LFS are pricey so I'd prefer to buy one from another hobbyist. I'm not having much luck on Craigslist. 
Also, how difficult are they to install? I'm hoping to find one that's basically put-together-then-plug-and-play like my canister filter was because I suck at mechanical details (hence why I don't use a sump -- confuses the crap out of me). Thanks!
Also, how difficult are they to install? I'm hoping to find one that's basically put-together-then-plug-and-play like my canister filter was because I suck at mechanical details (hence why I don't use a sump -- confuses the crap out of me). Thanks!