I know a number of plants that are happy with their EtaPro systems.

The biggest 'problem' with most performance monitoring systems, is that they don't actually DO anything by themselves (unless you have big $$'s and can invest in one of systems that communicates directly back to your control system). You need a site champion to look after the care & feeding of the system, and also review the results so you can put them to use and get some value out of the system.

I've worked with sites in getting packaged systems operational (i.e. EtaPro), and also put together simple spreadsheet (and some not-so-simple) systems that use data from PI or SQL data historians to look at the curve-based corrected performance as well as other performance indicators like compressor efficiency and condenser cleanliness. (I'm an independent consultant in this area.)

There's a wide range of systems available out there.

I've outlined some of things to consider in some of my newsletters - available free online at T2E3 - Newsletters Archive

Good luck with finding a system that fits your needs!