My son has been wearing cloth diapers for over a year already (he's 14 months old). I loved these Thirsties covers when he was tiny, and still like them. The PUL is not the thickest so every time I unsnap them, I'm afraid I'm going to tear it apart. However, the main issue is with the Velcro -- babies learn how to undo Velcro really fast. My son figured it out when he was 4 months old, so I had to either switch to covers with snaps, or install snaps in my Velcro covers.
The Thirsties prints are adorable, and even if thin, the PUL still doesn't leak.
Having different sizes (vs. the OS covers) means that you get a better fit, even if you will have to buy more covers within 5 months.