I love my USC. It's very accurate, manageable recoil, and reliable. I didn't like the grey color, so I dyed it black (cost: $8 for 3 bottles of black rit dye). Looks very different.
You can go the 30rd conversion route. I haven't heard of any problems with the modification, but I haven't done it because I don't like the looks. The mag-cinch is also my approach (20rds on hand). If you want to buy more than 2 HK mags, don't. At $40-50 a pop, your better off investing in the conversion.
Very soon we should have a modification to go to a UMP pistol grip and fixed stock. People are working on the design and legal issues as we speak.
Heck, the best part of "The Matrix Reloaded" was the UMP on the freeway. Now if I could just get the Hi-capacity magazine they used in the movie (about 800rds?)