I have to concur with dona83. I tried a Mini 9, and to be fair it was fun. I enjoyed the process of doing something "off road" that was a little tricky and felt like I'd accomplished something. But really, that's about it. The Mini 9 is usable, sure, but I would still go with an old 12" Mac, even if it's a bit bigger and heavier.
Realistically, there is no support for the Dell other than what some enthusiasts have put together. Getting actual support from Dell is a nightmare, just because they're a terrible company when it comes to customer service. My Mini 9 arrived with a damaged battery. It took literally 3 hours on the phone, being transferred from one department to another, most of the reps totally incomprehensible (phone lines had tons of noise too), and just got the impression that either no one knew how to help me, or would just rather pass it off than take some initiative. Of course, each of the reps asked me the same questions, like they were reading off a sheet. In the end I just gave up, and as luck would have it, the battery started working the next day. I'd put up with this if the machine was reliable, but it isn't. It feels cheap and especially with the constant workarounds required to keep up the Mac facade, I just don't trust it to keep working when I need it to, which is basically the main Mac selling point. As others have stated, it's not like it's even that much more powerful than a 5 year old iBook.
I'd say if you have the cash and you enjoy the process of making and modifying it to come up with a useful and unique computer (some people have even cut apple-shaped holes in the Dell's case to make it look like a tiny Macbook), go for it. But if you just need a tiny Mac for practical purposes, I don't think it's worth the hassle personally.