I've been playing tennis in this painfully hot weather and was constantly wiping my hand on my top to get rid of sweat. The handle covering was PVC, fake leather or some other cheap rubbish and I realised I needed something. I put the overgrip directly on top and it's much improved. I can hold the racket without fear of hurling it at the opponents now. It feels slightly sticky and slightly padded even though straight out of the packet it feels really thin. I don't think it's noticeably increase the handle size.
The only negative point was that the sticky tape was a bit short and narrow. It's not left the greatest of finishes but I can always replace it with something else if it comes off. (It hasn't come off yet after 3 sessions of play.)
I understand these overgrips will wear out eventually and for the price and performance, I'm happy to buy more.
Noob note: the strips come with a backing material which you peel off. I didn't realise this and failed to wrap it properly the first time as I'd left this backing strip on. *whistles innocently*