Yes I use it. I've explained in other threads, but it goes like this:
Every Saturday RPS post their "Bargain Bucket". If there's anything there I want, I buy it. If there isn't, I chuck either £4 or £10 in my Steam wallet.
The next time Steam has a decent sale, I use what's in the wallet to pay for it. This is an excellent way to convince my lady that I have some kind of (entirely fictitious) "gaming budget" that I never exceed. It also means that when a sale happens and I don't have the readies, it doesn't matter because I've already paid, so to speak.