Personally, I had a lot of little problems with CM7. The battery life was not that great for me, contrary to what everyone else has told you. At first, CM7 ran wonderfully. But after a few weeks, many of my apps would crash every time I tried to open them, even after I uninstalled a lot of things. I was also getting duplicate SMS all the time. It got to a point where I had no choice but to get CM7 off my phone. After I switched I no longer had these issues!
I switched to CM9 - it's beautiful and extremely smooth and 100x's better than CM7. I tried it out for about a day but the no camera issue was a big one for me as I use my camera frequently.
So, I then switched over to the next most stable ROM - Miui. There are two available here for the MT, I believe.. I can't remember exact names or version numbers. All I know is mine's the older one, not v.4. If I were to list my favorite ROMs in order of which I like the most, Miui would come in close second to CM9. I actually get great battery life with miui but I know this isn't the case for many people. Best way to keep the battery alive with any ROM is to turn off your wifi, gps, data, bluetooth, etc. and keep these things off as often as possible. If your girlfriend likes to customize the whole look of her phone, including boot animations, wallpapers, lock screens, etc all of these things are available right on miui. I can pull up tons of themes right on my phone without the hassle of needing to go online on my computer and download everything to my phone. It's a really user friendly as well. The themes are super easy to use. MIUI does not have an app drawer, however. It keeps your apps on the home screen more like an iPhone. But I use folders to keep my stuff organized.