While I haven't used them, my MIL has for my niece. She said they are a little pricey but she only uses them when my niece sleeps over her house. She thought they worked really well.
I bought a washable mattress cover (just the top, not over the corners) for DS's bed since his mattress was brand new and I didn't want to be washing the mattress cover all the time (he went through a stage where he peed through his diaper pretty regularly). Don't mind washing the sheets constantly, but I just hate taking the mattress cover off all the time!