As it doesn't have any kind of non-UPnP playback (that is, it can't play back local files), I haven't evaluated ArkMC (AppStore link; $2.99, Universal) in my UPnP bible listing and comparing all regular multimedia players (also with) UPnP capabilities.
As you will see from the pros/cons list below, you won't want to use it if you do need high-resolution support during software decoding. Otherwise, it's a decent app. Of particular interest is AC3 support during hardware playback of iOS-native files. If you, on your UPnP-enabled NAS server, have tons of MP4's (MOVs / M4V's) with only AC-3 audio tracks and don't want to add an iOS-compliant AAC track to them and want to stream to your iDevice, you should take a closer look at ArkMC. (And backup the IPA before Dolby makes Apple remove the player from the AppStore because of the AC-3 support...)
A filelist on my iPhone 5 (as you can see, there's no 16:9 screen support):
AC3 support, even during hardware playback
Hardware playback (including even Apple CC's)
Good MSMPEG-4 decoder
Can act as a UPnP control point (one that just acts as a proxy between a UPnP (file) source and a UPnP media render hardware); for that, it has proved to be excellent. Given that for example PlugPlayer costs $5 (and isn't even Universal) and doesn't support as many containers and video formats for local playback, this app is definitely a better choice, should you also want local playback.
No 16:9 screen support
No SSA subtitle support
Absolutely no support for local playback
Doesn't make use of the full resolution during software decoding on small-screen devices like the iPhone 5. An example resolution chart screenshot of playing back an MKV:
Unfortunately, this also applies to the Retina screen of the iPad 3/4. That is, you do NOT want to use it for playback on any high-resolution iDevice (any iPhone / iPod touch model starting with the iPhone 4 / iPod touch 4, respectively – along with the iPad 3 and 4).
Note that, as usual, you shouldn't pay attention to the AppStore reviews. In the US AppStore, the previous version has received two one-star reviews. Both are factually plain wrong: the player does play back files off UPnP sources and it does discover them.