It's very good - definitely closer to the PC version than to the first mobile game. For those familiar with the PC version, not a lot of stuff has been outright removed (there's no driver scouting, and I'm not sure if voting for rules changes is still there), but some things have been streamlined for mobile - like parts have fixed performance/reliability, parts bonuses are simplified, upgrading your HQ is done through a skill tree type thing, the engineer market now mirrors how the sponsor system works (small selection of applicants pops up each race).
Races seem pretty much the same - setup options are simplified, and there's no 3D car models (you see 2D icons going round a 3D track, although I think it looks quite nice), but apart from that the same driver orders/pit strategy options are there. Better than that, there's actually a few new features/improvements that aren't on PC! Like you can now buy off-the-shelf parts as well as building them, the game highlights the drivers who are interested in driving for you, you can improve a driver's stats individually, and the young driver programme from the first mobile game is back.
I've only went through a couple of races but imo it looks like they've hit a great balance between detail, and the ability to pick up and play. Presentation is excellent across the board, and it runs great on my iPod Touch 6th gen (sort-of equivalent to an iPhone 6). I'd recommend it. 👍 There's also an editor you can purchase which allows you to change team names and series rules, but it's about double the price of the base game.
EDIT: Also should point out the ruleset is much more limited compared to the PC version, you're restricted to options for refuelling, points system, safety car type, double points and making gearboxes/engines spec. On the bright side, the editor lets you change these rules for each series at any time. The editor is a good addition, it even lets you change any driver's stats/potential whenever you want!