I tried it. Not bad, but had some problems. Even though the hot keys are the same and I use them, the client ended up being slower for me and I had to switch mid-way through an event to try to avoid timing down. Round 1 I won 2-0 but would not have had time for game 3. I switched halfway through round 2 and still lost to time though, which hasn't happened to me since I was new.
Some more serious problems include not showing me what a morph is when I Treachery it away from my opponent. It does tell you the morph cost so I was able to deduce the creature, but you should be able to see the card face.
It also skipped my Temporal Mastery miracle trigger when I drew it off of Consecrated Sphinx on my opponent's turn. The Mastery was the leftmost card in my hand so I assume I drew it first, but nothing happened.
It also doesn't sort your library for you when you search it, which makes finding singletons a pain.
It also doesn't show reminder text for keyword abilities in the card popup, which would actually be nice for cube. I don't always know the details of older abilities and I assume given they have essentially unlimited space that they would include it. I did see a reminder text setting, so maybe I had that on wrong? This will require further investigation.
Visually the whole client is nicer, and the internal windows behave much better. Unfortunately some elements of the interface are too understated/hard to notice. The cards in hand/library/graveyard are much more subtle, which makes them harder to read at a glance. The queues suffer from a similar sort of situation. Nothing distinguishes them from each other in terms of font or highlighting, so it can be slow to read through. The font is also quite a bit smaller than the current client, which is bad for squinters. I don't normally have trouble reading text on the screen, but had to concentrate to do so several times with the new client.
Drafting is better. I like seeing the table order, and the way they let you arrange your deck while you play is improved. On the other hand when it comes time to deck build the area reserved for your sideboard is about three times the area for your deck, and I couldn't figure out how to change that. I also don't know how to save a draft deck, but maybe it does it automatically. I didn't check for that, but that would be nice.
I'm sure the program underneath is structured better, and having worked as a programmer I understand the intense amount of work involved in rewriting a code base and making sure it's well documented and easily upgradable. I would assume that MTGO users do not even see the biggest and most important changes, and the thing that most excites me about the new version is a greater ability to change and improve it.