Hipswnger2 pm'd me earlier tonight asking about manual boost controllers, and I replied only to find out his mailbox was full. This is information that is certainly available elsewhere, but there is a lot of misinformation and ugly threads to have to search through to find it. So, without further adieu...
The biggest issue is heat (which will cause the cat in the up pipe problem Hipswnger2 mentioned) but you need to understand why it happens and how to prevent it.
Your ECU is set for a certain amount of boost and fuel at different throttle positions. Say you have the throttle pressed 50% -- it'll boost something like 7psi with the stock boost solenoid. You put in an MBC and it'll allow full boost any time the turbo is capable of pushing that much air -- and your ECU isn't expecting it. So you're getting X amount of air in your cylinder, while it's only adding enough fuel for Y amount of air.
This results in a lean situation (more air, less fuel). Running lean equals hot exhaust gasses, and potentially blown piston rings or similar problems. It doesn't matter if you only have your MBC set to 13.5psi, equal to stock -- it's still going to boost higher at lower throttle positions than the ECU is prepared to deal with, and thus cause high temperatures.
To prevent it, you can do a couple things. Get a fueling computer (which is by no means perfect, but another topic of discussion entirely), UTEC or other management (which obvously costs more than you're looking to spend since you're running an MBC), or change your driving habits a bit and get used to watching your boost gauge. That's what I do (for now). Since the problem will generally be when you have the gas pedal partially pushed, whenever you're doing that, look at your boost gauge and make sure it's not peaking out. I generally try to stay under 7psi unless I'm giving it full throttle.
As long as you understand and follow all this you should be good to go.