I've heard mixed reviews, no not as goot as Sigma or Tamron but not entirely crap, stay away from them if you are on auto focusing cameras.
If you are using older manual cameras, they are really not worth it as superior glass can be found cheaper.
They would be acceptable for a young beginner on a manual film body, the price would tolerable in the event something unfortunate where to happen requiring replacement, but once out of the beginner stage it's just short of useless and holds absolutely no resale value.
If I wanted to take my nine year old neice out to take pictures of birds with with me, it would be a long phoenix tele on my AE-1 with a tripod. That can fall over and I won't be bankrupted.
Over all, steer clear unless you have a need to protect some more valuable glass that you alredy have.
That is my two cents at least.