It's a great lens. I had it for my crop camera for 4 years before switching to full frame. I used it mostly wide open, and it works well for portraits. The dust issue is overblown. It has a more limited range than the 15-85 or 24-105, but the lack of range doesn't matter much if you're carrying a tele-zoom anyway (i.e. 70-200).
Its weaknesses are build quality and flare resistance. It's not weather sealed, but I found that it worked fine for my uses. Unless you plan on exposing it rain and dust storms, it should be fine. It's not as tough as a typical L-lens, but it's not as fragile as the 50 f/1.4 either. It's in between. The lens is prone to flare when it is really bright outside depending on shooting direction, but I tended to compensate for it using exposure compensation and then fixing it post (but then your 7D's metering system is better than my old camera's).
I would skip both the 16-35 and the 17-40 unless you plan on moving FF soon (within a year or two). The 17-55 is more versatile than either of those lenses, and it is optically superior to the 17-40 especially wide open. Nikon's 14-24 is better than either 16-35 or 17-40, so I think there's a lot of room for improvement for the next versions of those lenses. It would be a shame to spend more for FF compatibilty and then have Canon release a much better version when you finally make the move to FF.