Good idea. I'll start negative w/ the two things I don't like:

1. Mute/Solo affects all subgroups/mixes.
2. Inability (perhaps my own) to route mixes to mono outputs and not stereo pairs.

Something that intrigues me and I haven't gotten yet - ASIO Direct Monitoring.

Despite my complaints, I'm pretty sure the main issue is my lack of time to really play and explore all of the features. I'm often on a time constraint and don't have the luxury of spending an hour getting headphone mixes together. I'd love to get a band in there w/ the time and patience to let me put TotalMix through the ringer.

That said, I look forward to hearing what others have to say.
Hopefully we'll all learn something.

One of the best tricks to learn the FF800 TotalMix GUI is this: Take some multitrack session you have already recorded and return each of those back to TotalMix as if you were going to analog sum or what have you. Plug your monitoring chain into whatever you want. Now open up the mixer and/or the matrix and start twisting the virtual knobs/sliders. Now you can practice panning the returns or setting up your groups or monitor mixes or whatever. Find out when the submix is useful and when it isn't. This can be done at home with a set of powered monitors whenever you've got thirty minutes to fire it all up. If you find a combination that is golden for what you do on a regular basis then save the mixer state as a preset.

My 2 cent contribution to the TotalMix Tip thread is to use the Matrix view to begin with!
The Mixer view is actually a great looking mixer once you first comprehend:
Upper Bank - is your hardware inputs
Middle Bank - is your "software" DAW outputs (which RME calls "software playback")
Lower Bank - is your hardware outputs

It starts to makes sense when you wrap your head around that part but unfortunately it's still not very intuitive as to how you patch those channels together....

If you want everything as mono channels then you have to make sure to "unlink" each channel pair in the options menu for starters.
Then you need to pull down each fader menu and check the connections you want
It can get very confusing!....but I find that the Matrix view is much easier to "cross connect" everything together.
Then once you've made a group of cross connects switch back to Mixer mode and watch the levels moving on the faders
Like Jack mentioned you can quickly see how a channel can be cross connected to any other point or multiple points or every point if you like!!
Once you experiment with what can be connected you will find out that you can create multiple horizontal rows, multiple vertical rows and multiple diagonal rows however you like.
One other tip that isn't really about routing but is just an interesting visual tip thing...
On the RME forum there is long running thread has all types of User provided "skins" that can be loaded to change the look of your Mixer view
It's essentially a new color bitmap that is copied into the default TotalMix folder and gets loaded in place of the stock RME bitmap rendering!
I use the Cubase "skin" but there are all sorts of interesting skins with different meter colors and button enhancements that makes it look "cooler" than the stock view!

Welcome to MindFire Academy

I use RME hardware for over 11 years, now. Totalmix was and still is a great feature I used for e.g. headphone mixes. Now, with the powerful control room features in Nuendo 4/5, I use it only for managing the I/Os, occasionally.
RME is a great company with a good customer service. All my RME gear is still working flawless. The first Digi96 and the Multiface 1 being the oldest I still posess. You bang them in and forget about it...from one PC to the next and next...Their stuff has always been a huge bang for the buck, but the sound of the newer Micstasy and ADI 8 QS is extremely good. We will go MADI with RME next year...

Good mentioning the control room feature in Nuendo 4/5 - it's another aspect of my setup I haven't had time to fully explore.

Between that and TotalMix, I get the feeling I have a beast on my hands.... now to get around to taming it!

Indeed, ... I have not gotten behind all new N5 features myself, yet, but between features like ContrRoom, expanded automation, new ADR PostPro features, MediaBay and batch export there is always something new to "discover" to improve ones workflow. With latencies down to 0.7 ms there is no need for me, anymore, to use direct listening through RME hardware. Reverbs are fast enough, as well, to deliver a decent headphone playback, too. With ControlRoom you can mix up to 4 separat, independent headphone mixes incl. sends and inserts... For my usual day-to-day work that is good enough.

Oops, back to MADI..
Although, the optical cables are less vulnerable then I initially thought, the connectors are just too easily unplugged for my taste.
Apart from the fact that opticals are also more expensive, a 75 ohm coaxial cable with a lockable BNC connector is usually very durable and comperatively cheap.
Last week I auctioned 20 various BNC-cables (1,5 m to 15 m) for 15 Euro... enough for the whole setup...