The Canon EF 28-105mm f/4-5.6 USM Lens often gets confused with the noticeably higher quality Canon EF 28-105mm f/3.5-4.5 II USM Lens. It is, however, a vastly different lens.
If you take a Canon EF 28-90mm II USM Lens and stretch it to 28-105mm, you get the Canon EF 28-105mm f/4-5.6 USM Lens. They are very similar on many accounts other than the obvious similar appearance.
Both have cheap build quality (plastic mounts, no distance window, wobbly objective ends ...). Both are light, small and inexpensive. However, the Canon EF 28-105mm f/4-5.6 USM Lens costs more additional than 15mm should in my opinion.
Both have a Micro-USM (Ultrasonic Motor) - decent, but not Ring USM. Neither has FTM (Full Time Manual) focusing. Both utilize a tiny MF (Manual Focus) ring on the end of the extending portion of the lens.
Using a decent (notch up from the 28-90mm II USM Lens) 6-blade aperture, the 28-105 shares the same lens' slow-for-28mm f/4 base aperture opening, moves to f/4.5 around 35mm, quickly to f/5 near 40mm and to f/5.6 around 60mm.
The 28-105 does not share the 28-90's very good magnification spec - turning in a relatively poor .19x figure at 1.57'.
From left to right in the picture above are the Canon EF 28-80mm f/3.5-5.6 II Lens, the Canon EF 28-90mm f/4-5.6 II USM Lens, the Canon EF 28-105mm f/4.0-5.6 USM Lens, the Canon EF 28-105mm f/3.5-4.5 II USM Lens and the Canon EF 24-70mm f/2.8 L USM Lens. The same lenses are pictured below extended to their maximum length.
The Canon EF 28-105mm f/4-5.6 USM Lens is soft at all focal lengths with the aperture wide open. I would say the wide end is sharper, but figure on needing f/6.3 or f/8 to get decent sharpness. Corners are soft from 28-35mm until stopped down to f/8 or so.
Even on a 1.3x FOVCF body, barrel distortion is noticeable from 28-45mm or so. Slight pincushion distortion is present by 105mm. The Canon EF 28-105mm f/4-5.6 USM Lens controls vignetting reasonably well showing up primarily at 28mm with a wide open aperture.
The 28-105 has a very nice focal length range that includes the normal, general purpose zoom range and more. But, I recommend, if possible, purchasing one of the lenses suggested on the Canon General Purpose Lens Recommendations page. Even the Canon EF 24-85mm f/3.5-4.5 USM Lens and the Canon EF 28-105mm f/3.5-4.5 II USM Lens are nice upgrades to the Canon EF 28-105mm f/4-5.6 USM Lens.