I paid 270 for this lens brand new after i checked tons of mixed reviews.
as someone else mentioned, this lens should be rated on 9 or above.
Focus is fast and tracks well... bit noisy and not as fast as a usm, but very usable for sports and birds photography, as long as there is enough light on the environment.
build quality... let's face it... you can't spec the durability of metal and environment seal of L series, however, polymer technology (plastics) has evolved enough to make us able to build car engines with it... so why complain?
benefits of plastics is the weight reduction and cheaper end user cost.
if i where a canon engineer i'll be thinking of building a ring type usm version of this lens