Been running it for four days as a primary desktop OS, and the stability is impressive. There have been some program errors, which is to be expected. I’ll keep posting if there are any big problems.
It is MATE 1.17, which seem to be mostly improvements under the hood; no big changes to the interface. It’s just like using 16.10. This is a Good Thing.
There is a “new” Software Boutique (re-built from scratch) on its way, but it didn’t make it for Alpha 2. 
Last night, after leaving the system on doing a caja-driven backup (I was having problems with grsync), which took a very long time, the machine started acting hinky, like not being able to recognize me as a member of the sudoers group. I rebooted to a black screen with a blinking cursor.
So, long story short, I whipped out my trusty USB and reinstalled.
Everything is OK here. It works like it should do.
But I have to say there is long delays at startup/poweroff and even at logging off a user.
It may be normal at this point. I guess it will improve eventually.
Yes, startup and shutdown time also changes for me. A week ago startup took over 2 minutes and shutdown was fast. Right now startup is fast and shutdown takes a minute. This also happen with 16.10 testing.
I've been using 17.04 as my daily for a bit over a week now and it's been very good for the most part. I haven't had any issues with startup or shutdown time variances but a couple little issues have carried over from 16.10. Most notably the icon sizes on the top panel aren't always correct (Inhibit applet, Steam, Clementine, etc) but no big deal breakers. Here's an image showing the icon issue with the inhibit applet and Steam.
Well. I had another crash (my own fault, messing with GPartEd) and tried to install. The USB reported a kernel panic. Hmmm. So I burnt a DVD. Same kernel panic. Created a new USB with today’s (17th) release. Same kernel panic.
This is while the disk is booting, every time.
The .iso, of course, runs perfectly in VirtualBox.
I assume you load the iso from your home folder to your vBox guest install. If so, this sounds like a (dvd) burn problem.
It cuold be a DVD error, except I did use Brasero, which verifies the chexksum of the .iso. Besides, I did use both Unetbootin and dd to create a bootable USB. No luck. I am going to check my RAM.
After 14.04 started giving me big problems, I (finally) managed to install 17.04 again, and, after a little problem updating grub, I’m in smooth sailing again.
So far, UM 17.04 is almost boringly stable!
I really like that in Caja the back/forward mouse buttons finally work even in icon mode. Is really cool. The dark theme is nice too. Good work guys.
Well stated Frederick. So little to do 
My standard payload of must have apps are all working with out incident, and I’m still able to make my preferred GUI changes. All in all, quite impressive.