I've been looking at the H&K Compacts in 9mm.  Why would I want to buy one?  Valid reasons only please.  Thanks,
W

HK's unconditional lifetime warranty. Put 10K rounds through it and then send it into HK for warranty work. They'll fully rebuild it and send it back to you damn near new for only the cost of shipping.

Unlike Glocks, the chamber in the USP and USP compact is fully supported, so your chances of having your weapon explode in your hands are very, very slim to none, and Slim just left town.

I don't worship any manufacturer, and when the HK USP series came out in 1994, I was not super impressed. It seemed like HK had realized that the only way that they could compete with other manufacturers was to injection mold 1/2 the pistol.  

I eventually wound up with a USP.45C
Reasons:
1) Supposed to be very corrosion resistant
2) supposed to have fewer problems with case rupture than Glock.  Rated for +P
3) the HK Compact fits my hand very well
4) polygonal rifling for the tech weenies
5) Very good reputation for durability and reliability
6) tactical light can be hung off rails, although tactical light costs more than it should
7) I like the exposed hammer over a striker.  
8) reasonable weight.  

Ultimately, it all depends on whether the HK feels right to you.  If it does, I think you will be very happy. If it doesn't, there are plenty of other excellent pistols out there.  

This is a no brainer. Most people who buy automatic's soon find that they are somewhat of a pain in the ass to disassemble and clean. The H&K USP Compacts are very simple to disassemble and reassemble due to fewer parts and better design structure.