I would say
http://www.noobz.eu is the MOST reliable place to get the downgraders since this is the the website maintained by the crew who actually made them.
Downgrader downloads in other places are basically mirrors from the ones you will find there.
As for reliability during the process, MOST of the bricks (if not all) are due to inexperienced users who neglect to read the instructions properly.
As long as you know what you are doing, the downgrade shouln't go wrong. Read the instructions 20 times if necessary until you fully understand every single step, then you will be ok.
All in all, you should be aware that:
Any downgrader carries some risk of permanently damaging your PSP. We've done everything we can to remove as much danger as possible, but if you choose to use this downgrader, you are accepting all of the risk involved. Nobody else will be held responsible for any damage to your PSP, howsoever caused.
