If your usage is purely HTPC, Mac may not be the best(or I must say cost effective). You could build HTPC from components or go for prebuilt once like
Zotac Zbox or
Dell Inspiron Zino HD. If custom hardwares are chosen, you could get better performance/features for the buck. (Though when it comes to aesthetics, I did not yet see any PC case that beat Macs).
If you use any Mac as HTPC you must be aware that you will be sacrificing :
a. HD audio(DTS MA/Dolby TrueHD) bit-streaming.
b. Blu-ray support.
Here is
review of Mac mini as HTPC from Anandtech.
However, if you are looking for a computer + HTPC, Macs are great. I do have a Mac Mini as my computer which also happens to be my HTPC. It is hooked to 50" plasma tv as its sole display. I do use Plex as my media organizer which can be completely controlled by Apple remote. It is fine for me as I currently do not have a Bluray collection and I do not have a receiver that decodes DTS MA/Dolby TrueHD. Once I have them, I might be migrating to a custom built windows based PC for HTPC
