I personally think the biggest issue with Mach4 is communication. No body seems to know the current status, or where the various plugins are at.
If you only ever post here on the forum, you only ever see the problems being discussed with very little if any input from any of the Mach guys. At least over the Yahoo group you get some input from Brian. Running my own business I understand you have to prioritise where time is spent, but I'm sure they could at least spare an hour or two a week to post on here and provide some idea of what's been going in the background.
Even just a thread only Mach guys can post in to act like an informal blog, and give a brief roundup of what's been happening over the past week.
As for the plugins, it would be good if details were a bit easier to find. USS/ESS I keep hearing about, but unless you dig into their forum, details don't really exist. As has been mentioned CS do have a plugin, but all the public details point to it being well out of date. The only person who seems to be keeping on top of things is Steve@PMDX giving updates when required.