Rob, the 24mm would be a great choice as well, and I'm almost certainly going to end up with one in addition. (I've still got one foot.

) Though honestly between the Mk I and Mk II, I would really recommend the Mk II because the flexibility of the new rotating tilt & shift mechanism is amazing. I shoot back & forth between horizontal & vertical framing all the time, and need to flip from horizontal to vertical shift at the drop of a hat depending on what kind of perspective control and/or stitching I need for the composition. Tilt, of course, for normal purposes anyway, would usually be vertical... though I can see some artistic potential in having angled tilt. Nevertheless, the ability to quickly flip the axis of shift & tilt with respect to each other is crucial. The fixed relative tilt & shift is one of the biggest reasons I never went for the original 24mm TS-E. (Yes, I know the Mk I can be adjusted, but not mounted to the camera, in the field, in 2 seconds, with gloves on, and no tools!) I can't be positive enough about the mechanical aspect of the new design. (Though the optics are equally fantastic from what I see so far.)
Re: filters... it's just a compromise that has to be accepted. You know what kind of a filter shooter Darwin is

and he has so far found the 17mm to be more useful without filters than he first expected. Personally the only filter I use is a circular polarizer and I definitely do miss it. But there are many circumstances where filters are merely beneficial, not necessary. I have always loathed the Canon 17-40mm and for some reason never bothered with the 16-35mm, so one of my heavy workhorse lenses has been the Sigma 12-24mm ultrawide zoom. It likewise has a bulbous front element and can't take any filters, so I've lived without in that case. (The same is true for anyone shooting the Nikon 14-24mm, which I'm also strongly considering obtaining.) I've made many, many compositions with the Sigma that do not suffer from the lack of polarization. Think of it as icing & cake... and the polarizer is not the cake.

I shoot ultrawide compositions a lot for landscapes and now for some architecture, and the first & foremost issue is can I even get the composition I want. The second issue for me is can I get a high enough level of IQ, and this is where the Sigma 12-24mm was letting me down. Though I love the flexibility of that zoom range, and despite its positive aspects like very controlled CA and rectilinearity, it just doesn't have the level of corner-to-corner sharpness I want to see in my 5D Mk II files. (Nor did it on my 5D, but I was willing to live with it at the time.) I consider the filter aspect a third ranking issue after the first 2. Though I know for many people it may rank higher, just as is the case with their dissatisfaction with the Sigma and Nikon ultrawide zooms.
Not trying to do a sell job here

since Canon's TS-E lenses have always been something of a specialty series, and the 17mm variant is even more specialized. But for ultrawide shooters it is quite literally a unique tool that, in my mind, justifies its only significant limitation: not supporting filters. (Well, and having to use a piece of cardboard

-- see Mac's blog entry.)
Mac, does this line up with your motivation as well?