I started my web dev business building sites using Joomla - actually Mambo before it forked, and I 2nd the updating issue. There's no reason a well-planned update should break live sites.
I think the Joomla project has been surpassed by other open source CMSs since, and that at least some of the key community members that used to work on Joomla have moved onto other projects. (That's a total guess, and I don't mean to insinuate knowledge I don't have - just that the open source dev community is perpetually in flux) The benefit of this is that there are several other viable alternatives - the drawback is that the market (read: open source CMS projects) is fragmented and potentially less stable.
Since Joomla I've done lots of small WordPress sites, a Drupal build, and now work with SilverStripe and Concrete5. I actually won't put clients on Joomla sites anymore - and have turned down Joomla work on several occasions because it's usually a bad situation where the client has gotten into a Joomla setup that was highly customized and is now broken. I'll migrate people out of Joomla, but won't work with it, fix it, or put people in it.
Ease of usability may be the Joomla claim, but after working with a half-dozen clients to train them on using it I never saw it. I never ended up in a situation where the client could do everything they needed through the front-end admin - and training them on the administrative panel and modules and plugins was always much more information than they needed or wanted.
I work and talk with several usability folks - the running joke is that CMSs suck. They all do. They try to do too much. My only comeback is that I try to use the least-sucky for the situation and keep expectations realistic.
If you are interested in the "next" thing to come along in the CMS world I see frameworks as "it". Developing a Ruby on Rails site it probably out of reach financially for many nonprofits, (I would like to learn, but have not yet) but SilverStripe has an interesting model where there's an open source CMS (BSD license I believe) coupled with a php framework called Sapphire. It's a little more costly to set up as it will most likely require more coding, but the end result is a much cleaner admin interface because you start with a basic CMS and build what was needed where it was needed into it. There's none of that "go here to do this, but go to this totally unintuitive other location to do this other related thing" with SilverStripe - the interface looks like windows explorer, and the needed fields are page specific. It's got a great versioning system too.
If you're looking to build a site that really makes good use of the "front-door"/inline admin interface, checkout Concrete5. It's interesting, and I'm having a lot of success adding custom code and mingling it with the CMS without invalidating the upgrade path.
For what it's worth, I say the same thing about Drupal sites too. To my way of thinking they're both CMSs built by developers who try to satisfy (too many) non-tech people without thorough testing, usability battering, or the ability to just say no. (They are open source, and have been around for a while.)
Good luck,
Matt Souden
matt@msouden.com