I've only been home brewing for around 3 years and haven't progressed beyond kits as yet. Whether I will, I really don't know. As I'm still working I'd probably struggle to find the time to spend a whole day doing an AG brew, but as long as I'm getting decent results from kits, then I'm happy for now.
My first few brews all suffered from 'twang' and I was on the verge of giving up, but things have improved considerably now due to gaining more experience (thanks to the advice found here) and acquiring a brew fridge (which I think is a big factor in making better beer from kits). I tend to stick to the premium kits - I'm not going to quibble about a pint costing me a few pence more to make if it's a better pint!
If I had to pick three things that have improved my beer I'd say 1) the fridge, 2) being really careful when I transfer from the FV to the barrel before bottling for batch priming - I now start with the barrel almost laying down so that I can rest the end of the tube on the inner side of the barrel, meaning the beer flows without any splashing and therefore putting minimal oxygen into the beer, and 3) (a great tip from
@terrym) - as I use PET bottles, once the beer is in the bottle I gently squeeze the bottle to bring the beer nearly to the top and then cap. It leaves the bottle looking slightly distorted but within a day or two, it's filled out completely and I have less oxygen in the beer. This is obviously not something you can do with glass bottles!
EDIT: Actually, there are four!!! 4) Give everything time - you simply cannot rush a brew and expect to get good beer at the other end!
For anyone who feels they're not getting decent results from kits, if you haven't done so already, I'd say try a much darker beer. These seem much less susceptible to any twang. I followed a recipe on here for St Peters Stout, which included adding freshly brewed coffee and priming with soft brown sugar. I'm not normally one for darker beers, but this is superb (I'm still drinking it) and it's easily to pub standard.
I have Mangrove Jack's Pink Grapefruit IPA secondary fermenting at the moment and it was one of those brews where everything just seemed to go right. No issues at any stage so it will be interesting to see if this is reflected in the beer quality! I'll post a review in the appropriate thread in a couple of months.