Career mode is awesome and has been pretty much 99% of my play time in this game. I started at Manor and managed to pull them right up to 3rd or 4th fastest team in the first season. I'm driving for McLaren in season 2, currently leading the WDC due to several Mercedes fuck ups (twice failing to set times on dry tyres in a drying qualifying session, couple of 1st lap retirements, and a Hamilton engine blow-up). Assuming normal races for the rest of the season, Hamilton could well beat me to the title. I just can't keep up with them.
My old Manor team kept their pace for about half the season but massively fell back later on. Interestingly the biggest jump in season 2 has been Toro Rosso, they are very close to Mercedes but their drivers suck something special and they have barely registered a podium.
I've actually been surprised by the testing programs, I thought they'd suck but I find them very useful in warming myself up for qualifying and the race, and most importantly learning where I'm shredding the shit out of my tyres.
Also the AI is surprisingly good, I even find myself blaming the AI for crashing into me, but when I check the replays I see it was my fault generally. Sometimes crazy shit happens though, like in Japan Kvyat had an issue and drove super slow, causing yellow flags in any sector he drove through, but none of the AI drivers would pass him causing a huge traffic jam. He then parked in on the track causing a Safety Car but was also disqualified for 'Parking in a dangerous position' or something.