The problem with most kits is that beer needs a certain amount of hop flavour, even non-hoppy English bitters, and the canning process seems to kill off the hop flavour. I think that's why Coopers Stout is highly regarded, because stouts are one of the least-hoppy beers out there.
In my experience it's always worth dry hopping (or making a hop tea) the canned kits that don't come with any additional hops. The Coopers kits seem to benefit especially from this, even the lagers.
The only kits I've come across that have come out really well without any additional hopping are the Muntons Premium kits, Midas Touch and Smugglers. They need a bit of time in the bottle but were superb when I did them. I must revisit them sometime, to see if they are as good as I remember them.