Hoya HD Clear works great on the 17-55...and, yes, it is easy to clean as advertised.
For those who question whether a filter would actually protect the front element from a blow, consider at least an extension ring. I've seen seemingly innocent bumps dent the threads of unprotected lenses. A filter, a hood (threaded or bayonet), or an extension ring would help prevent damage to the lens' threads.
In the real world, something that would dent unprotected threads, if you had a filter on, would jam the filter on. You'd then have an expensive repair to remove the filter, repair and / or replace the filter threads, and maybe even replace the front element if the filter can't be removed without damaging it..
Without the filter, just burnishing out the dented threads would probably be all you'd need.
And, with a lens hood, you'd either have no damage or would just have to get a new lens hood.
Seriously: use hoods for protection except for cases where you need eye protection or weather sealing for certain L lenses in really nasty weather conditions. And, even then, still use the hood. <i>Especially</i> then -- the filter is going to make flare worse, possibly much worse, and the hood is going to cut down on flare.