I've had a lot of experience with Joomla of late, as I've been converting my 'other' site (Broadway Baby) to it.
On the pro side, it does give you a pretty comprehensive foundation as a CMS, and saves re-inventing the wheel for many common functions. As Matt says, there are also a lot of extensions for it, so development can be pretty rapid even when you want to build something unique or obscure.
Where it is a royal pain is getting your head around the way it wants you to work, and doing anything off the pre-determined plug-ins (and eventually you will, because the default situation is unlikely to fit all sizes). For a procedural programmer like me, getting to grips with the MVC design pattern that Joomla uses can be a struggle at first, and making even simple modifications to the code can be challenging (especially as you'll want to do it in a way that isn't broken come the next upgrade.)
You should also bear in mind that the benefits of the many hundreds of plug-ins and components come at the cost of compatibility between these functions, and as a fair chunk of them are freebies, you're unlikely to get industrial-level support from the developers when you run into an issue that only happens with a certain cocktail of extensions.
It is worth sticking with though; but expect to get your hands dirty in the code as a vanilla Joomla site can be very uninspiring (and there are probably better, smaller, options for such a simple site anyway).
Pete
