This is a Festool forum so not sure what you're expecting to hear.
Anyway, I use a Fein Dustex 40 in combination with a Dust Deputy. I chose the Fein because it's a very reputable German brand and more importantly a lot cheaper than the Festools. (Designations in the US are different, but I think it's the Turbo III for you guys) So here are my findings after about a year of use.
Performance wise it's every bit as good as a Festool CT in suction/cleaning performance. No noticeable difference with the CT33s I've used in the past. There's also TRUE HEPA filters for the Fein and the "self-cleaning" felt bags, slow start power tool outlet and a faucet at the bottom to let out liquids.
If the vac is plugged in, the power outlet always gives you power. The vac doesn't need to be running or be in the auto position. It can just be on off and the outlet still gives you power. Some people like this, some don't.
Noise wise, I'd say the Fein is about 3db less noisy, meaning just slightly noticeable. Where the real difference comes in, in my opinion, is that the Fein sounds more civilized whereas the Festool has a more annoying whining sound. No big deal, unless you're standing next to it all day long I guess.
My Fein came with either a 1 or 5 micron filter I can't remember. I ordered a TRUE HEPA filter for mine so the original is in the trash. I also fitted it with a 36mm Festool hose. (Fits fine and snug, no adjustments whatsoever needed).
Never tried the original one so I can't comment on how it fits with tools. (It too went straight to the trash with the original filter)
So be prepared to possibly pay for some extra enhancements. (Over here in Europe we don't get a HEPA filter standard but I believe you Americans do) So by the time you've bought some additional stuff you'll be almost at the same price point as a Festool. Something to consider.
The vac has never tipped on me. The six swivel castors make it a very stable vac that's extremely easy to roll around.
And lastly, the Fein comes with metal vac tube attachment, not the plastic connecting set you can get for the Festool. The problem is storing these is a nuissance. Which brings me to the most important, and not to be disregarded easily, advantage of the Festool CT's: storage! I can't count the times I've wished I could store some systainers on top.
So, all in all, the Fein is a very good vaccuum, plenty powerful and probably the quietest I know of, but I constantly struggle with the question if I shouldn't have gotten a Festool CT26-36 instead, because I really love the storing systainers, boomarm & workcenter idea. I hate how my hose is always on the floor gathering dirt and I'm always stepping/tripping over it.
The only, but very important problem with the Fein, is it's form factor and shape. It's a big vac (holds over 30L) so when full it's heavy, and there's simply no way to store your attachments on it. Basically, you get a good vac but none of the Festool niceties in terms of storage and attachments.
Also, you might want to have a look at Starmix vacs, they're the vacs that many many other manufacturers have rebranded. Like Metabo, Bosch, Hitachi, Mafell, Spit ... Also very high quality German tools with the same performance.
Hope this helps you make your decision. Either way, you'll get a quality vac.