I've had good luck with New Relic* although on first glance I'm not sure if it offers as comprehensive a monitoring service as this system.
Tracelytics has two big features that New Relic doesn't have:
- Support for service oriented architectures. We'll show you what called what, and how the caller affects the callee's performance.
- More flexible filtering throughout the app. We let you filter on URLs, controllers/actions, custom-defined apps, even memcache keys or SQL queries, and carry these filters around with you go from page to page. It really helps isolate a problem with, for instance, a memcache call that only originates from one URL.
Now, obviously I'm biased, but I think that these two can make a big difference, especially when you're trying to track down a performance problem for the first time.
P.S. I posted a related question on Quora recently, maybe you'd like to weight in: http://www.quora.com/How-is-Tracelytics-better-than-New-Reli...
In the case of vanilla PHP, the concept of controller and action is less well-defined, so we provide a function that you can use to log controller/action information you have in whatever routing class/function/layer you have.
In other words, try to explain to some manager that the development & server administration needs some special tools for monitoring. They might think they need to hire someone else to fix that weird issue when the load spikes.
So, this means there's only one option: having an open source stack to do it and set it up in-house.
For now I'm just running the NewRelic free version on two of the slices.