Hmm. I agree with some of the complaints you have - the overworld would have been better with more things to do, and driving can be a bit tricky thanks to the sluggish handling of some vehicle types and the crowds all over the winding roads (Jak 3 improved on things a fair bit - more stuff to do and the new areas have vehicles / mounts that are easier to control without running into people all the time).

Personally I found the first game to be harder overall than the latter two (although Jak 2 had some particularly hard missions, some of which were due to camera/control issues (THAT TANK SECTION). Jak 3 had only one or two parts I found frustrating, one of which was a [non-optional] Pac-Man -like minigame) - I found collecting some of the power cells to be a pain, but most of the boss fights were pretty easy.

The first three games have some issues with checkpoints, although they aren't as annoying as some other games I've played.

I found the plot and gameplay (for the most part) enjoyable throughout all of the games (although I haven't played The Lost Frontier properly due to a glitch that prevented further progression). Jak 3 has a story issue at the end I wasn't happy with, thankfully Jak X fixed things.

If I were to rate them, I think I'd say: Jak 3 > Jak X = Jak 2 > Jak and Daxter

The Ratchet & Clank, Jak & Daxter and Sly Cooper games are some of my favourite games on the PS2 (with the exception of Secret Agent Clank...). I wouldn't describe them as '"kiddy games" - I think they'd be appealing to a variety of age groups.

'All 4 One' and 'Q-Force' / 'Full Frontal Assault' aren't really representative of the rest of the series (in gameplay as well as story).