If you have 4 external preamps you could use 16 ins without using SPDIF, or 18 with SPDIF. I can't say I've ever felt 18 channels to be limiting. I understand if you dont have any external pre's though, I'm still working on that myself we're borrowing gear for our "golden channels" at the moment.
The only reason we're buying two is for tracking drums printing through UAD Studer and some EQ / Compression, and for the option to track a full band at once if need be. There's a few people on Gearslutz going on about why you would want to commit, but I'm so excited for that. Nothing like making the record just sound immediately better, it gets the job done faster and keeps the band happier as the record sounds more "finished" as they track.
I just can't believe how cheap this is considering UA's track record of just completely missing the boat pricing wise. If that thunderbolt card is $500 or less these things are going to fly off the shelves. I'm just so excited as this opens up Pro Tools HD levels of latency and workflow without having to invest in that damn antiquated card structure. I completely understand the benefits of DSP in the tracking stage, and even with this interface there's still things that I won't be able to do but I think this one might have AVID a little scared.
I see this as their competition to the Omni, it's really what the Omni should have been. Having enough I/O for small studios like my own but with professional conversion and high-quality analog components. I was really interested in an Omni when I was contemplating going HDNative for my mixing setup, but I am planning to expend to a hybrid mixing environment and it was just to limited in outputs. Then there's HDX, I think its fair to say that most people on this forum have absolutely NO NEED for a HDX system, apart from the luxury of monitoring with plugins. Which to be fair is a drop in the water on those new cards. I think a few of the smaller studios that are making good money that would have jumped on a HDX1, have a more affordable option which is more tailored to their needs.
I might sound like I'm gushing now, but for once someone has released a product that fits my needs perfectly. The proof will be in the pudding though, this thing needs to sound / function as good as it looks on paper.