I would say the only pro is the f1.8 bit. Everything else is a con.
On a crop body (like our friend's 450D) 50mm is a bit of a weird focal length.
Its slow and noisy to focus... yes, really slow and its focus accuracy is poor too.
The build of this $1 hat is really $1's worth.
Basically, if you want a cheap way of seeing whether wide aperture crazy DOF effects are fun, get one. If you like it you'll probably want to get the better built, better quality and far more expensive 50mm f1.4 variant instead.
Most people one way or another have the "nasty fifty" (as I prefer to call it) for a little while then either reject the idea totally or spend a bucket load on the 1.4 instead.
Me, I'll never own another again