I also mean browsing from my couch. The mouse works perfectly on the top of the back of the couch and is at least 10 times faster than using a remote.
The main reason I tested it was for our anime club where we often don't know what to watch so having posters and stuff is a great help in deciding but in the end it was just so unpractical. For example the built in mediaplayer was not very good, you could use an external one but if you did that you would need both mouse and remote because the media centers have bad mouse support and external media players like MPC-HC has bad remote support.
The interface speed problems where compounded by the fact that we used an underpowered computer (Atom/ION based nettop) but it was slow even on my main computer.
But I digress, some people prefer a good looking interface over a fast one and others prefer the opposite.
Take note of that OMM devs, for the standalone program make sure you focus on practicality and speed over looks. You already have the plugin for those that prefer the later.
I don't know how much you've tried messing around with it, but the default MediaPortal player is customizable. I didn't like the idea of an external player when using a media center, either. However, in the MediaPortal config, I was able to tell MP to use MPC to render video, audio, and subtitles, and video playback has improved greatly since then. Actually, last night I tried using ffdshow for video rendering, and so far, I'm pleased.
Even still, I do think that a standalone OMM designed around a keyboard and mouse is a must, since I know a lot of my friends that would feel silly using a remote to watch anime on their laptop screen. I do feel that if you want to use a remote, media centers are the way to go, even if the technology isn't perfect yet.
I know that the MP-TV Series plugin has some nifty features that allow users to browse large collections quickly. For example, being able to scroll through a list with a page up/ page down function (MyAnime has this). Also, if you already know the name of something you want to watch, you can use the number-pad on a remote to scroll through alphabetically (MyAnime does not have this, but they may implement it soon).
I doubt I'd ever use one though since I haven't seen enough redeeming points to warrant a change.
The one thing I really like about this plugin is that it's not invasive. So, if you wanted to try it, it's not like you would have to alter your collection in any way. I've been setting up an HTPC for awhile now, and most media managers require a certain setup in which each folder has to be named a certain way, and each file has to be named a certain way. So, oftentimes, I would find myself renaming thousands of files to get them recognized in a media manager. Even MP-TV Series suffers from this fault (well, it has to, because there does not exist a database of CRCs of shows that aren't anime

). All you have to do is install it, show MyAnime where your anime is, and enter your AniDB account info, and then it does the rest. If you don't like it, you just uninstall it, and everything is as it was before.