I'm tempted to get one. I know the site says "!!! NO SOLDERING NECESSARY !!! Just open your player and plug it in." But do you know what's involved? I'm not afraid of opening a piece of electronics (well, especially one with no amplifier in it, so no big transformers/capacitors), but I'm just...
The documentation for the UDP-203 doesn't mention MHL, and I happened to still have the Roku Streaming Stick that I got with my BDP-103D. I just tried plugging it in, and nothing happened, which is what I would expect from a non-MHL HDMI port. Sorry.
If you want it, I now have my 103D re-boxed and ready to go, with all the original accessories (though I swiped the power cord from my new UDP-203, so you'll get a nice new untouched power cord). The remote was used twice - when I first got it, and when I was shopping for a new 4K UHD TV...
They've said multiple times that they don't have any streaming apps yet. Seriously, they've not been shy about it. They wanted to focus on getting the shiny, spinning discs right - you know, the main job of the device?
For projectors that do 4K but can't do HDR, it allows them to use the Rec.2020 colorspace (which covers more than Rec.709). Some other UHD-BD players (like Samsung's and Panasonic's) don't let you strip out HDR metadata, so you have to choose SDR + Rec.709 or HDR + Rec.2020, which doesn't work...
If you want one (and you're in North America), I'll probably be selling my BDP-103D soon, now that I have my UDP-203. It's in perfect condition, and I have all the original packaging, cables, manual, etc. They're not being sold new anymore, so second-hand is going to be the only way to get them.
It's not quite accurate to say it's launching with full Dolby Vision support - the SoC has the necessary logic to support it (which I assume Dolby has at least signed off on, otherwise I doubt they'd have mentioned it in the marketing materials), but it still needs firmware updates to implement...