We stayed there once as part of a "stay and cruise" package (basically, 3 days at Disney then a Princess cruise from LA up the coast to Vancouver) - it wouldn't have been my first choice simply for the location, but there was no choice. All in all it was fine - the hotel was clean and the rooms were a typical hotel room. Service was OK. I didn't like that we had to use the ART to get to and from the park - it's a good system and I know some people don't mind it, but I hate having to line-up for the shuttles at the end of the day.
My suggestion would be to take care NOT to get a room on the ground floor -many of them look right out onto the pool area and it can be pretty noisy, even at night (there's a hot tub, too, so even if it's cold out you'll still have people out there). If you're on an upper floor, they have some rooms with little balconies that are nicer - a little more spacious, and it's nice to be able to get some fresh air if you want. We actually had one of the larger rooms initally, but it was a smoking room and so we requested a change as the smoke smell was really strong. We wound up on the ground floor - the noise was the lesser of the two evils, but it wasn't ideal.
For a non-resort property, my first choice would be something like the Carousel on Harbor, or the Candy Cane Inn (which is walking distance but also has a private shuttle). But if it was a choice of the Clarion or nothing at all, I'd definitely be fine with the Clarion - it's not fancy, but it does the job.
Oh - and for the on-site restaurants, you'll be OK with the garden-y one (sorry, can't remember the name), but I'd avoid the sports bar themed one. We had dinner there our first night, and it was NOT good - terrible, actually, and my DH found a long hair in his entree. Yuck!!
