I've got 2. I've modded both of them maybe 3 or 4 times each, to play with their filtration and flow. Running it all foam is nice, but can clog up quickly and slow down. using better biomedia works well too. right now, my preferred way to go is to swap sides, so the biomedia is in the return chamber instead of the intake. the way to do this is to get a large intake strainer, and center it in the return chamber, then fill with small media around the strainer. now you can slide the return tube into the strainer, and it's surrounded by biomedia. it's a better use of the space, because now the entire intake chamber is open, and you can fill it with foam or pot scrubbers, etc. I like to stack blue foam in there with a piece of denser foam on the top.
I'd also love to run this with 100% pot scrubbers, but i haven't yet. I may replace some of my bio media with crushed coral, to see if I can help my snails out.
now, as far as that little clear tube that is the outflow/flow indicator goes... it's the weak point of the whole filter. It IS replaceable. It HAS and REQUIRES an O ring! if you dissasemble, be sure to reinstall the O rings. I have removed the red brass flow indicator from both of my filters. In my experience, they don't act as flow indicators, they simply restrict the flow, and give detritus a perfect place to settle. I'm tempted to drill the whole tube out, but haven't tried that yet. I have broken several of these. Tom is always happy to send them out in the end, but it takes a lot of back and forth every time.
When I first got these filters, I took one look at the tubing and swapped it with Eheim tubing. It's always worked well that way. More recently, I needed the extra flexibility that the corrugated tubing offered, and have been using it. It works Just fine, and is compatible on one end with other ehim fittings and tubing, so It's easy to add an intake tube or spray bar of your liking. I was actually glad to have the stuff this time around.
One of the best features of the filter is the low voltage power supply. Being able to quickly disconnect the low voltage plug is great. Way easier than jimmying a plug out of a socket. I only wish they had done the same for me for the tubing... something like mini-eheim disconnect valves would make this SO much easier to deal with.
I also find it's really hard to prime in some cases. It would be awesome to incorporate an inline prime cap, like what's discussed here
http://www.aquariacentral.com/forums/showthread.php?t=188096
guess that's all I can think of.