I've installed three or four alarms, one on my own vehicle (remote start and pager), and a pager (no remote start) on a coworkers car. I'm by no means a professional now, but I do pride myself on the details. Hidden wiring, good placement, and no problems. I'm not going to say I'm just as good as those professional installers. I installed the alarm in my car because I'm very familiar with my car's wiring, and I've got some experience with electrical. In my opinion, one of the main differences between an installer and a really good installer is the details. Anyone can follow a wiring schematic. Connect the Alarm green wire to the car blue wire, etc etc. But, knowing where to place the components, and where to access particular wires, is the important part. I hate looking under the hood of a car and seeing the alarm siren sitting in plain view, and obvious alarm wires sitting out. It's sad how many people butcher installs with poor execution.
When I tackled my first alarm, I had no real idea what I was doing. I had nothing more than the instructions for the alarm, no vehicle wiring schematic or anything. This was pre-internet too. It was a late 80's Firebird, so not overly complicated wiring. I just made a plan of attack, laid everything out, tested the wires, and installed the thing. Admitteldy, it was a lot less complicated then the alarm I installed in my own car, but still, I managed it. Funny story, when I was finished installing the alarm, I put everything together, the dome light was on. I freaked out, thinking I wired something wrong. I took the whole thing apart to realize I left the dash dimmer switch in the up position.
The latest alarm I just finished was a pain, since I don't really know anything about Honda wiring ('89 Accord) so it took me several hours to figure it out.
Nothing I couldn't handle though. Haynes Manual and online wiring schematics made it easier. Still, when you look under the hood, you can't tell any alarm wiring from the factory parts. The only giveaway is the blinking LED and the antenna, even that's disguised though.
The point of this rambling is, if you feel sure enough and don't mind getting really familiar with your car, go ahead. As others have said, if you're hesitating, it's probably better to get it installed. There's a reason alarm installs cost a lot (relatively). It's not easy work. Shoving the shock sensor anywhere isn't going to work very well, and leaving wires exposed for a thief to cut won't do much to help either.
Nothing bothers me more than people that think shops rip them off. They can do much better than those certified mechanics.
I had a customer come into my work, order some brake parts, and needed some clear hose. I asked what the clear hose was for, and he explained that he needed it to bleed the brakes. Then, he asked me how to bleed the brakes. I felt like asking him for the make, model, colour and license plate of his car, so I could avoid it. I don't need his "My First Brake Job" failing when he's behind me in traffic. Pay someone to do it, it'll probably cost less in the long run, and if something fails, you can point the finger at someone other than yourself. That's why they get paid the big bucks.
I have a lot of respect for professional installers. Keep up the good work.