2x Extender for Canon

Venu Bharadwaj wrote:

i'm looking for a 400mm lens - mainly to use for cricket and fast sports like F1 shooting !! (i'm not a pro)

i have canon 50D with 18-200mm lens (f/3.5-5.6)..50mm f1.4

i'm extremely confused on to buy a new lens (300mm or 400mm) or buy extender 2x for my current lens (18-200mm) - i know with with 2x f stop will be f/11.2 @400mm - as it will be for outdoor - i reckon it should be ok?(i may be wrong here)

the doubt/confusion i have here is

• is Extender good with zoom lens or recommended for only prime lens?

• is the 2x Extender good for my lens? will i be able to achieve good, sharper images?

• Canon 2x Extender is out of my budget - need to limit the budget within £250

• other than Canon, read some good review about Kenko 2x Teleplus PRO 300 DGX - is this good for my lens and camera pls? or any other you recommend ?

• is it worth to buy Extender

your view/thought will be greatly received...

Thanks.
Venu