Comet apparently has somewhat of a downside when it comes to guns, in that it's top gun is barely an upgrade from the Cromwell's top gun, which in tier 7 will cause you problems when against heavier armour, people recommend gold rounds but I'm not going that route. But it's not a bad tank at all, as with any tank you just need to be aware of your strengths and weaknesses. The Comet is a good machine in the right hands.
Another complaint you'll hear is that it's slower than the Cromwell therefore it's criminal, but I'm not with that crowd. Certainly the Comet is slower than the Cromwell, but the Cromwell is ridiculously fast (can keep up with some light tanks and sometimes even outrun them) and while the Comet is slower, it's not slow by any stretch of the imagination, it's still fast. Historically they purposely designed the Comet to be slower than the Cromwell because the speed of the Cromwell had downsides, in that you could potentially wreak your own tracks and I think the engine as well, so they made the Comet slower in order to better prevent self inflicted damage.
Centurion Mk. 1 will be a whole new world though, and the Centurion 7/1 is apparently "legendary".
I'm not a fan of British heavies. I've only been down the German line for heavies, I have a stock Tiger. Most my tanks are mediums (Sherman Jumbo, Sherman Easy Eight, Cromwell, Comet, VK 30.02 M Panther, and the Panzer III/IV), I have the one heavy (Tiger), and I have a couple tank destroyers (Jackson and Hellcat, used to have the Stug III that I enjoy but I needed the credits so I could start my Comet off with a crew at 75% training and I didn't want to sell the Cromwell).
