I only briefly owned a Mini. I always appreciated the external design of them and one day I impulse-bought an open box 2012 base model from Best Buy for about $450 or so ($600 new). I was running an HP tower with an i7-2600 CPU with Windows 7 Media Center and Plex Media Server. I loved the idea of replacing it with that little Mini, but I knew that the tower I already owned was more capable, so I couldn't justify it and ended up returning it.
But for those arguing in favor of a NUC, I still don't see it as having a clear cut value/performance advantage over a Mac Mini. I just took a quick check at Newegg and I'm seeing the following:
http://www.newegg.com/Product/Product.as...-_-Product
$350
i5-4250 w/Intel HD 5000 graphics
PassMark score: 3462
CPU, RAM, HD (SSD only?) user-upgradable
smaller case, but less attractive
2 USB3 ports
external power brick
no built-in IR
no HD included
no RAM included
no OS included
2014 base Mac Mini
$500
i5-4260U w/Intel HD 5000 graphics
PassMark score: 3651
No upgradability
larger case, but more attractive
4 USB3 ports
power supply all internal
built-in IR
500GB HD
4GB RAM
Mac OS X included
If you're willing to live with the lack of upgradability, the base Mini might still be a good deal compared to a similar i5 NUC.
Now, if you are looking to just use this as a client, you may not need an i5 at all, and you can get a Celeron-based NUC for as little as $150. But if you just need an XBMC client, even that NUC may be more expensive than you need to spend. If you can live without 24p and some HD audio and VC-1 support, my Fire TV works pretty well, costs less than $100, and doesn't require any hardware upgrades. The newer Android TV boxes will hopefully be even better.