I've bought non genuine chargers back when I had a Powerbook with no problems, but you do hear some horror stories - albeit very rarely. You still wouldn't want it to be your house on the local news with a fire engine outside, mind you.
I bought mine from a local computer shop - i.e. someone with a tangible reputation to defend. It wasn't the cheapest one out there, but it wasn't as much as apple wanted (I seem to recall back then they were £60 or something ridiculous) £28 doesn't sound so bad for peace of mind, to be honest. There is a surprising amount of electronics in a power supply, as I discovered when I cracked my HP one open accidentally.