I'm currently looking at the Zend Platform as a php acceleration tool rather than eAccellerator. I briefly looked around at ionCube and I've been using eAcc for a while. When reading the reviews, I found that eAcc and ionCube supposedly have problems handling high loads - which sort of defeats the point IMO. The cost of Zend is pretty high, so I'm wondering - 1) is it worth it. and 2) what is your experience about performance between different accelleration platforms? Zend has alerted me to some problems I had with quoted strings in my templates that were causing php errors that I hadn't noticed, so it definitely has side benefits as well. That and the download accellerator looks pretty cool, but I don't serve many downloads currently. The compression engine is pretty useless to me since I already have implemented a compression solution wherever I can.
I haven't had any problems (that weren't resolved from the developers) with eAccelerator on high loads. I have one server that serves up roughly 500-1000 hits per second (all PHP based) and it's fine with eAccelerator.
I find eaccelerator to be excellent. Not sure what is considered high load but I know that I can tweak my eaccelerator and have not had any problems. You just have to make sure to give it enough ram to work with.
I have had excellent luck with EAccelerator, I have not had load problems, on a few of the sites I have put it on. If money is not a big issue Zen is a great way to go.
My thinking on Zend is that eAccelerator did just as good a job of increasing speed (if not better - I don't have any real stats). Zend has some great features, but at $1000/machine or $1500 if you have 2 cpu's, I'm just not sure about it. As you experience growth, it really means a big expense to deal with every time you add a server. I'm glad to hear you guys with busy sites haven't seen any of the load problems - that was my big worry.
I've benchmarked Zend vs. eAccelerator (and some others) and eAccelerator was faster than anything else (by a long shot). Personally, I wish PHP would take eAccelerator and roll it into the default PHP code base. Even if it's a compile option you have to specifically enable. It's such a good feature I can't think of any reason someone would ever want to run PHP without it.
I have to agree that Zend optimizer neither helps in extreme loads nor it speeds up code more than eAccelerator. Actually, eAccelerator speeds up code much more than zend optimizer, according to most of the benchmarks. I have used all and my preference is APC. Works the best under extreme loads, plus it has some nice online stats available including hits and misses too . APC cache is the most rapidly developed PHP accelerators and since its developers are the well known ones (some php developers), there are more chances of it to survive longer. I am not sure but eAccelerator development has been real slow lately.