I am definitely only an amateur, so you may wish to take this with some salt.
I went through the same dilema a little while ago. I went the stepping ring route (I did already have polarizers for each filter size I own, and UVs, and thought that things a getting a tad silly). My experiences are,
I have found that on Ebay, stepping rings are relatively cheap (in comparison to filters!)
I personally have a 72mm lens, (and a couple of 58s and a 52), and I knew I wanted to get or at least hire the EF-S 10-22 (which I have at the moment, so I can see whether I would want one permanently, or hire a couple of times a year....).
So I decided that I would buy some stepping rings, 52->58, 58->72 and 72->77 (whilst I was about it), and then bought the ND/grad and colour grads that I wanted in 77. There was about a £10 difference in cost.
Obviously though I am stuffed if I now get a wider filter lens.
Bear in mind though, that the 10-22 only supports 1 filter at 10mm. (it says this in the manual, I couldn't see why, so I tried it, doh, must have been asleep).
On the photos that I took to check the lens and the way it worked, the corners are missing when I had the UV filter, and polarizer and grad. However, it is still a brilliant lens. This is with 3 filters on, that the corners are missing, but as I am considering trying to get some more panoramic like shots, I am probably only missing around 10% at most of the area I would want. If it were my own lens, I might do-away with the UV filter, and have the polarizer on permanently (hmm, that might make things darker, but for landscapes... longer shutter times are possibly not too much of a problem), but this is a hired lens, which I could not distinguish from new.
My understanding is, that the slot-in square filters, are less likely to have this issue, as they are much wider than the lens (did I see 110mm somewhere?), however, I believe there are two types of holders too, a single, and a triple, in which case the triple might still have an issue. But this paragraph is just what I have seen around, it could be complete rubbish.
I can't really comment on the full service, as I haven't sent the lens back yet (but I don't expect that to be the cause of any problems), but lensesforhire.co.uk is where I hired this lens from, and so far the experience has been good. Arranging for the hire of the lens was simple, I selected Royal Mail delivery, and it arrived by 1pm the day before the hire 'starts'. I even got an email the day before that to say that the lens was sent, and the tracking number (it is also a good reminder to expect the lens, as I had placed the order some time ago!)
With the square filters, I believe that you can rotate the holder, but this means that all the filters are rotated by the same amount. Thus, if you have a polarizer, with a grad (or two grads), I think that you might be restricted in which angles you can employ. Again, this is what I think is the case, it could be rubbish.
On any of the other lenses (52, 58, 72), which are 17mm at the widest, the stepping rings cannot be seen.
Edit:
Just seen this thread, post 18 might be relevant if you go the Cokin route,
http://www.talkphotography.co.uk/forums/showthread.php?t=128779