my take was .. I am very very new to photography apart from Point & shoot ..and it was only a few weeks ago I bought this camera (which I love but its hard to work out etc ie M ) ... so I considered the more expensive options, but still couldnt justify to myself to buy a more expensive lens .. I dont have the money hanging around too, who does with a three year old in tow !!! and I was so undecided whether I would get the use out of it and wanted all the other assessories like the bag, a UV lens etc .. that I thought walk before I run.
So when offered this, second hand .. I thought what the heck, although even new i was considering the lower end prices as I didnt want to spend $k and then think what the heck have I done ..
my advice ... if you are really keen on doing photography, havent the spare cash to spend on really expensive lens .. buy the cheaper 3rd party ones, and if you like what you get, or feel the need further down the line to get a more expensive one .. you can sell the "cheaper one" as they seem to hold their prices - check ebay out to see what I mean, and upgrade then.
all I can say is .. this lens does far more than I thought and it blow my mind when I focused it last night on the mountain across from our house and I could see the ppl in the ski-lifts !!! talk about cool or what 