Has anyone tried mining with visitors' computers Some sites like http://www.bitcoinplus.com/generate allow that.

But can javascript take advantage of GPU?
You should have a look at this question about web miners.

In general, web miners use JavaScript and WebCL in order to perform operations on the GPU.

However, it might not be as profitable as you think. Considering that most people have very poor GPUs. At the moment if you'd have someone with a decent 200MHash/s card on your website for 24h, you'd basically earn 3 cents. You can check how much you could earn by visiting my calculator. All in all, you're better with ads.