Warner was scouted to the Australian test team as a t20 player so it has definitely changed the game. As everyone watching the test today saw, the Aussies second innings was basically a t20 match with warner making 55 off 29ish balls and smith making 59 off 41. I think it's a good change though because it has brought a new stratergy and technique to test cricket which can make it much more interesting

I don't think that 20/20s have had as much of an effect as some believe.

I think batting aggression has been on a natural progression throughout the history of cricket and you have eras where it takes a noticeable shift. I think back to guys like Viv Richards and Dean Jones and then the effect that Sri Lanka's assault style batting in the ODIs had on the batting in Tests and then you look to the way that guys like Hayden, Gilly and Sehwag batted in the late 90's and into the 2000's.

20/20 is another one of those things that up the ante again where players can score very fast when required but unless you have good technique and can play proper cricket shots you will eventually get found out and exposed and not make it in Test cricket. Have a look at Mitch Marsh who is an absolute freak of nature in the shorter form but his technique just doesn't hold up at Test level. Dave Warner's Test career really took off when he put away those expansive drives and swishes outside the line of off and now most of his boundaries come from pure technique and timing to punch the ball through the gaps.

I think in the teams like the West Indies that 20/20 has had a negative effect on Test cricket as they chase the $ instead of a Test berth but if you ask any aspiring Aussie State cricketer where they want to end up I reckon 99.99% of them are chasing a Baggy Green which is why so many still opt to go to England to play County cricket.