I have tried rafting twice, once in Sg. Kampar (Perak) and the other in Sabah.
There are various vessel sizes, from 4 pax to 10pax. The smaller ones are fun as it is easier to manoveur, and more dramatic when it dives!!!
How dangerous it is, will depend on the current flow. If its raining season and the river is swelled, then you bet it will be fun and could be dangerous, more so when the vessel overturns at Grade 3 dive hehehe. The danger is hitting yourself on the big rocks, or getting stucked in one. However, this is very rare and the last one I heard was in Pedas, Sabah.
As for fun factor, it is awesome. Those that have not tried it, should give it a try and you will be hooked
Will try to make it, if I am free on that weekend.
Cheers