I've got that 28-105 (nonIS) 3.5-4.5 usm II and I don't rate it against my 18-55 kit lens!
The 18-55 is just sharper, and less CAs. The 28-105 can really smear bright areas across dark bits (sky 80% across lampposts at 50m). Its results are fairly good, but dont expect a leep from the 18-55 because it isn't, you have to kep away from the wide open end of the aperture range to get sharpness and it goes a bit soft at the 105mm end of the zoom too. It was a BIG step up from the kit lens that came with my EOS500N 7 years ago, but the modern kit lens is really quite good. I reckon the 18-55 kit lens is able to put slightly higher resolution onto the centre 2/3rds of the sensor than the 28-105.
It is a nice lens to use, the proper USM and the fulltime manual focus are good and feels nice and smooth. The build quality is reasonable, but the extending end of the lens is still a bit wobbly (nowhere near as bad as the kit lens though).
The 28-105 I find gives a nice zoom range on the 400d, but it would be nice to have a little bit more wide. I also have a 75-300 and can get better results at the 75 end of that than at the 105 end of the 28-105.
My thought is if you've got £200 for a new lens then keep the kit lens and accept its reasonable enough, but beg, borrow or sell your soul for the rest of the cash to buy a 70-200 F4 L from kerso while the canon offer is running . . . £280 for L glass, ooooh