I have been trying out some AJAX Libraries these few days...
1. XAJAX (for PHP)
Its cool, but lacks documentation (you have to look at the script and see some additional stuff you can do). Its also easy to use, but currently lacks the functionality I would like. If you want to use this, I suggest you download the one from CVS (since version 0.2 will be out soon) and version 0.1 lacks many features. Supports unicode. Its much simpler to use than SAJAX.
Its very easy to use, and its compatible with many programming languages, PHP, ASP, etc.
But again, it also lacks functionality. Does not support unicode unless you modify the script a little (don't use the js escape function). Cannot pass multi-dimensional array. I like SAJAX.
Of course, the two ajax libs above are not comparable to prototype. If you really want a really powerful ajax (and easy) library with PHP, you have to wait, or write one yourself. prototype.js is hard to use because it lacks documentation.