I've only done around 2.5k battles but I have to say this line is probably my favorite so far.
I've gotten as far as the Charioteer at tier 8 which is my new favourite tank – you get a fantastic 105mm gun from the tier 10 centurion, which has overkill penetration on standard APCR (268) and decent pen with the 480 alpha premium HESH (210), and is accurate and fast-firing as well. It's based on the Cromwell chassis so it will regularly do 52km/h, and it has a turret, so with a rotation device you can also play it like a (very fragile) medium tank.
I would thoroughly recommend this line to anybody even just for the Charioteer (let's not talk about the stock grind) but the rest of the line is full of gems too. The only real disappointment so far was the tier 6 Achilles, which felt exceedingly meh. And of course at tier 10 there is the wondrous Shitbarn of Doom to look forward to.
Anyway. Just felt like recommending this line because the Challenger and Charioteer have reminded me that this game can be great.