First diaper I "test-drove" was the Flip. This diaper has a waterproof shell that you place an insert right inside, then close with snaps. You can use the snaps to adjust the size as needed.
Here is Brenna in her Flip. (I love this picture because it's like she's the calm in the center of a brother/dog tornado.) I tried to take better pictures of her and the diaper, but she just wasn't having it. The diaper is a little bulkier than a disposable but her pants still fit her just fine. It just makes her look like she has a little more junk in her baby trunk. :-)
I really liked the Flip because it was easy to use. To change the diaper, you just open up the cover, remove the dirty insert and put in a fresh one. I never had any problems with leaks - even after an extended period. I haven't put her in it overnight yet - once I do I will probably put in two inserts for extra absorbency. The Flip seems to be the best economical choice out of the two I've tried because you can re-use the cover. The covers can be purchased for $13.95 and inserts are $4.95 a piece. I used four inserts with Brenna and only had to use a new cover after her BM. If you could time it right, you could easily use just one cover a day and just swap inserts as needed. The Flip system also has biodegradable, disposable inserts available for travel or scheduled poos. :-) I've used these and found them to do just as well as the cloth inserts.
For another really great review, check out Rachel's experience with Flip!