Does anyone have any experience with any of the following?

Afterburner
APC 2
php Accelerator 1.3.3
Turck MMCache
Zend Performance Suite

I've heard really good things about MMCache and php Accelerator in particular but haven't ever had the time to install any of them. I can't remember where I saw it but one example was Nuke sites being sped up 400%.

Are they worthwhile? If they are, what is the best way to take advantage of them? Would XOOPS benefit from native support?

I'm quite happy with my site's speed but even just doubling the throughput would be impressive!

I use Turck and it is very good. At it's best it actually seemed to be even slightly better than Zend. Probably what brought about its downfall as word is that the developer of MMcache was headhunted by Zend and as a result MMcache has been an orphaned project for over a year now.

Might be worth having a look at eAccelerator which is a continuation of the MMcache project.