I bought a Clifford Matrix alarm cheap via a friend's employee discount, thinking I would install it myself. I mounted the antenna on my rear window and strung it through the headliner with no problems. However, after several looks at the various multicolored wires under the dash and realizing I had no relays for my locks, I gave up and paid $200 at Best Buy (failure to provide receipt) to install. The alarm ended up costing more than if I had bought it outright and installed for cheap.
Overall the alarm was installed well. I have a complaint about where they put the siren (very obvious to the eye when you pop the hood, and its wire is in plain view making it easy to cut), and the fact that they mounted the trunk sensor in the hood (redundancy -- popping the hood would jar the vibration sensor anyway, and now I can open my trunk without the alarm going off at all). I'm going to have that fixed sometime. Otherwise, the install is clean and I've had no problems.