Getting a little sick of the one star reviews for this product. The first review like this was November last year so there's really no excuse now for people to buy the thing and then give it a one star because they didn't notice it is for use with the PS3 Slim only. It is now stated in several reviews as well as in the manufacturer description.
Anyway, I bought this the other day and I'm really happy with it. It takes your PS3 Slim from two USB slots to six. The only restriction is if you want to charge more than three controllers at the same time. To do this, you need to invest in an AC adapter that plugs into a socket on the top of the unit and then I assume you could charge six if you wanted to. There are also LED extensions on the unit so you can still see the power and HDD lights on the front (although they are a bit dimmer).
I think this is a really handy thing to have if you own a PS3 slim. Before mine arrived I was constantly unplugging things to make room for others and I never was sure if it was okay to just pull out a USB stick as you shouldn't do this on a computer without "ejecting" it first. Now I don't have to worry about it any more.
If you have a PS3 SLIM and want more slots, there's no reason why you shouldn't buy this.