Although the 50mm f1.8 Mark I uses a lot of plastic, its built is much better than the Plastic Fantastic (version II). I owned version II for a while, before upgraded to the 50mm f1.4. Both versions of the 50mm f1.8 focuses very noisily, if you are used to the silent USM lenses, but the Mark I feels more like a lens than a toy. Both lenses are optically similar, but version II has newer lens coating. I actually prefer Mark I which I think is sharper at the corner in large apertures. Overall, it's a nice little lens that despite the age, performs really well. If you don't care about the 2/3 of a stop slower, there is little advantage to upgrade to the 50mm f1.4 USM.
A note on the XSi body. Though it's a Rebel, the focus is very quick and quite accurate, very similar to the Canon 30D I had. However, the file shows its age compared to the Sony NEX-5N. Even at low ISO, the Sony images are cleaner. But, it's better than the T2i. Yes, the older XSi has better low ISO image quality the newer T2i. A shame, really.
Startled Birds - Canon Rebel XSi & EF 50mm f1.8 Mark I @ f2.8. Click for larger.
Playground - Canon Rebel XSi & EF 50mm f1.8 Mark I @ f2.8. Click for larger.
Cyclist - Canon Rebel XSi & EF 50mm f1.8 Mark I @ f1.8.
Bridgepoint Hospital in Distance - Canon Rebel XSi & EF 50mm f1.8 @ f4.0