For the £70/£80 or so they go for I think they are great value. Mine is very sharp up to 150mm, especially stopped down a bit to say f5.6 but there is a noticeable drop off after that (@160mm to 210mm - although still decent, especially stopped down a bit - again 5.6 seems to be a sweet spot on mine at the longer end too).
AF is a bit noisy and slightly slower than some modern lenses but still decent speed compared to some other screw drive Minolta lenses I have used like the 75-300 Minolta 'new' version which is very slow and noisy!
Main problem is purple fringing/CA can be quite bad in certain situations, so not ideal for a jpeg shooter.
I guess there is sample variation with an old lens like this (last made around 25 years ago in the late 1980's) as the photos here look very good wide open at 210mm:
http://www.dyxum.com/dforum/a58-test_topic98055.html
If you can find one at a dealer with a warranty that will at least avoid the ebay lottery on old lenses like this (mine was £75 from Cameraworld with a 6 month warranty and a 7 day return option). If you do buy on ebay make sure the description specifically says no fungus, no oil on blades, etc - that way you can return it if it is not as described. Try and get one with the hood included too as they are more prone to flare than modern lenses.
On the other hand, the Tamron 55-200 is a similar price new (£89), comes with a 5 year warranty, is much smaller and lighter and is probably better for longer range stuff. I have both and use the Minolta for animal and people portraits and flowers/plants etc but take the Tamron when I want to travel light and for landscapes etc. For the price the Tamron is also surprisingly sharp stopped down a bit (although it's only f4 up to about 130mm, the Minolta is obviously constant f4 throughout).
http://photo.net/equipment/tamron/55_200_Di/
The Sony 55-200 is very similar to the Tamron (rebadged version??) and can be found for £99 new (white box, split from kit) and has the advantage of a non-rotating front element during focusing.