I Currently use this lens.
I have it on my camera all the time. The lowest exposure you can go with the camera + lens is f4 and highest f22 , which is great. ( for 20D )
The IS is a useful feature - if your shooting images which you can't/dont want to use a flash for. It's also useful for photographing something which you need to hold the camera still for. I dont use it much as it uses a bit more battery power and i don't like the noise it makes! Non-the-less its good.
55-250mm is a good range, i can get close enough and just far enough away for most things. It's nice and quick and the AF is good, but again i use MF simply for the practice!
Great image quality for the price, i picked mine up brand new on ebay at £145 inc delivery.
To answer your last part of the question, the best use for it - well generally, the best use is everything! You can use it for quick things such as motorsport or simply for photographing the sky or an ant.
Overall, a good lens to use
Hope that gives you some assurance
