Let me come back to the "personally used" question later.
I purchased the add on suite for BuddyPress through Andrea and Ron for StudioPress, so I am set for life with that as well.
I mean my first big site effort was a site for my college and I just couldn't fathom having fellow classmates having to organize and direct everything site related through a programming savvy person.
I should be clear, too, that as far as I know and have experienced, I get the artwork with StudioPress and ElegantThemes, so I have enormous amount of latitude to change the look of a site. I'm a visual person so it can be hard for me think outside what I'm seeing, but I have had to put a ton more effort into learning Photoshop to make a nice website than anything to do with PHP or the like.
As far as plugins, well what I'm telling you is that I think - I have ALWAYS thought - it would be impossible for their plugins to be original material. And now we have at least one confirmed report of this.
And let's talk about "quality" for a moment. If the software they sell you only gets updated while you are paying them exorbitant amounts of money every month, then that's a problem. More than that, it means they don't have automatic updating in their software so they will make you go and download updates and install them manually every time they update something. Now with StudioPress? I get an email and a flag in my dashboard to upgrade and I don't need any current valid credentials to get my update.
All of the great theme makers have plugins or use plugins commonly available and will help you get them working or troubleshoot them. I don't generally think to go to StudioPress with not-related-to-studiopress questions, but I absolutely can go there and get help, especially if it's related to how a plugin is jiving with my theme. And someone will tell me if I would be better off with another plugin for my purposes and/or StudioPress compatibility.
And the only reason you think they have a large collection of quality plugins is because they said "we have a large collection of quality plugins". Nobody I know has ever - ever - said they got things there they couldn't get anywhere else. I think the clientele they tout sort of speaks for itself in wanting software but not being connected to the WordPress community. For all the money they want, that doesn't include installation. I find it hard to believe you'd get some kind of better concierge level service from WPMUDev... it's the same post it in a forum and wait for a reply.
I mean who here hasn't followed search engine results to WPMUDev that for at least three clicks I thought I had reached a page of their site where they were providing a free solution to my problem and then just when I thought they were doing something in the WordPress way, threw up the pay wall for me to pay insane amounts of money to them. I've blacklisted them on my google search results, they give nothing back to the community for free. If they have a plugin in the repository I'd be very surprised. It took me like 50 times to figure out that the .org in their name means nothing, and the wild goose chases were how they scammed people into paying. Every time I found my site for the first 50 times I thought they were only charging for certain select premium services. EVERYTHING is premium with them. They have free plugins (with registration) but I have been led on to their site too many times with pages containing wording that indicated to me that the solution was available to the public only to be very surprised when it came to the paywall.
I mean this is is all just my opinion, but if you move around that site, can you honestly say that you think a company that spends THAT much time and energy in marking and crafting these [seedy] long trails for people to waste time following day in and day out so they can make a sale... I've never seen any WordPress site be so high pressure on sales... does this not make you want to go wash your hands after you land there? Knowing what else is available in the WP community, you knew there was a better way and a "righter" way and that's why you made the post today. Oprah would tell you: you knew!!
I will tell you that we really need a non-coder's WordPress chat room. The one thing I find in the chat room is that the guys know to write amazing code from scratch and I typically want to solve my problem with a plugin and they typically want to tell me how to solve my problem with 1 line of code. It's all good though. But it might be easy to perceive that it's not so beginner-friendly.
Regarding Multisite or BuddyPress? I have done them both a few times and they both pretty much rock. I'm about to do a forum site and not sure I'm going to use BuddyPress. A client of mine turned me on to this Mingle Forum plugin and it's pretty freakin' creamy and smooth. They have a mingle social networky plugin too, which looks to be, in my opinion, a preferably smaller dose of social networking that was just right for my tastes. I don't know what your goals are for the site, so I don't know if a forum was a cornerstone of your project - but there's also that Simple:Press which looks very nice and a lot of people use. The main thing with multisite or BuddyPress is both of those solutions is that you need to be clear on what they aim to do because they're not necessarily what you'd expect from a multiple-site or social networking package. I actually reinstalled a complicated site recently because I couldn't deal with the way permalinks currently work in multisite for the first site - it puts a /blog/ there in the permalink... and with other parts of the site, having anything in between there was really wreaking havoc for the system and what I was trying to achieve.
So bud, get started on another thread and tell us what you are thinking of building and what you know you have organized and what you need help with finding for other building parts. You ask 5 people what plugin do they use for this or that, and you can get 5 different answers and they all rock.