M. "Doc" Skala tries things so you won't have to!
Overall, and so far, I'm not too impressed.
User interface: Facebook was bad for overriding standard UI behaviour, but Google Plus is worse. Nearly everything has stuff that pops up when you "hover" the mouse cursor over it, and in many cases that's the only possible way to interact with the things. There's basically not one standard HTML link in the whole system; forget about using tabs and the back button. This is not a Web site; it is an application that happens to appear inside what looks sort of like a browser window. And it's an application that breaks all the rules of desktop UI.
I am aware of research indicating that (for instance) circular menus are in some sense "better" than the ordinary kind. That's great as long as you understand the context: those studies are really asking the question "If an entire computer environment worked THIS way or THAT way, which would be better?" That is not the question Google faced when carefully designing their Web site to prevent it from looking or working like a Web site. The question they faced was "If an entire computer environment works THIS way, should our part of it work the same way, or should we override everything to make just our part work THAT way instead?" They answered that question wrong. But at least they didn't do the infinite-scroll thing like Facebook. That's a small mercy.
The bizarre user interface has an important consequence: it's confusing and nearly impossible to navigate. I never know where I am. Sometimes, it appears that all updates from people I've put in a "circle" appear on my "stream" or whatever they call it. Other times, I seem to be only looking at updates from one "circle". When I'm looking at a friend's profile, how do I go back to my own? Maybe click on "Matthew Skala" in the upper right? Maybe click on "+Matthew" in the upper left? Why are there two menus named after me - do I need to be reminded of who I am right now, so that I won't be confused when I log into all the OTHER Google Plus accounts for my multiple secret identities? Why is neither of them exactly, really, a menu as I know it, even though they appear in what a standard UI would call the "menu bar"? What does the little gear do? What about the box near the top that looks like a search box - is it really, or will things I type there be immediately posted to some or all of my "circles"? Will clicking on a person's name cause a pop-up about that person to appear, or take me to their profile, or make them my friend with benefits, or open a video chat with them? And why is the answer to that different on this page from what it was for an identically-appearing instance of the same person's name on some other page? Some of the answers to some of those questions can be found by hovering the mouse cursor over the objects in question. Others require more elaborate trial and error. I still haven't figured out what a "Spark" is; I read the help page for that and it was even less informative than one of Facebook's and then I couldn't figure out how to get back to the main site and had to close the browser and log back in again. The general confusion stems from the fact that standard user interface conventions, which were not broken, just weren't good enough for Google. They had to do something new and different. Just like with Orkut, remember Orkut?
On openness: much has been made of this idea that you own your data, the system doesn't own your data. It is nice to see, though I haven't tried to use it and am not sure how well it'll work, the thing that lets you download a ZIP file of, for instance, your own postings. But it sure isn't so "open" if you have an email account other than GMail! When a friend tried to invite me onto the system in the first place, he entered my correct email address as the person to invite, and then several hours later still having heard nothing, I dug through my old password records to find the GMail account I'd created years ago, and sure enough, Google had in its infinite wisdom directed the invitation messages to that GMail account I never use, which they had apparently found because my real email account was used as the verification address for that one.
I'm not the first to comment on the unpleasantness of Google Plus sharing data with GMail. Combine this invitation business with the fact that there seems to be no way to send Google Plus updates to any email other than a GMail email, and no way to forward GMail email to email elsewhere... the whole thing sure looks like a bid to use Google Plus to force people to use GMail, and just because I'm not paying a fee for either service doesn't make it not "tied selling." Even Facebook does not go that far. They have their own internal email system but will send notifications off-site. Google Plus uses GMail, period. ETA: Turns out it is possible to forward GMail to an account elsewhere after all; I'd just been looking in the wrong place for the relevant configuration. That makes this point less an issue.
All that said: I'm reserving judgement. I said I'm not too impressed with the system at this point but that doesn't mean the system is bad. Very much will depend on the user culture that develops, and that is unknowable at this point. They get many points just for not being Facebook... though that will eventually wear off and Google Plus will have to stand on its own merits independent of what it is not.