I've been debating about purchasing a MINI PC vs Building, sadly one of my HTPC's just recently took a swan dive. So do I purchase something like the ASRock Vision HT for ~$700+ ($800), Build or Repair?!
The only potential drawback to these MINI PC's as an HTPC is lacking of an internal 'TV Tuner' option. However, since Cable DVR's are very common place now the 'TV Tuner' need is getting smaller every day. I have Verizon FiOS and the newer black DVR's and a HTPC to record requires a CableCARD and PCIe CableCARD e.g. Ceton InfiniTV 4 but then you run into oddball things like "Copy Once."
The Pro's & Con's are all from their size, but ~$700+ is a huge price and IMO reduces the demand. The only complete system in this article is the Lenovo Q180, the ASRock Vision HT 821B requires some form of OS and sure there's 'free' OSes but Windows 7 Home Premium OEM 64-bit will add an additional $100.
Any of these listed can be used as a simple Desktop replacement. The workaround for storage is to either purchase a large capacity 'drive' (SSD or HDD) that can 'fit' or some form of external storage including an external drive (USB or eSATA if applicable) or Windows Home Server or similar network storage device.