I don't think its that bad of a pack personally. Whilst it doesn't make sense for the game's setting, the Beetle GRC is great fun to drive and is actually usable on the surfaces its supposed to be used on. Not like that rallycross Veloster from Forza 4.
The old Baja-modded Beetle is a niche choice, but is quite fun for just trekking across dirt tracks and cruising in. Great attention to detail on the car model as well.
The 850 is a car that I never had in Forza 4, so I was quite pleased to have it here. Very fun to drive despite being on the heavy side, has lots of body mods and makes a great noise. It's safe to say I wasn't disappointed with it.
The 2015 STI is the biggest surprise for me. Initially I wasn't a fan of the styling, but after using it in the game its actually starting to grow on me. Feels just as planted and fun as the other STIs which means its pretty agile and good for cross country stuff. I found it interesting they gave it more of the traditional Boxer rumble when the real car supposedly doesn't have that any more - then again, its hardly a bad thing.
I have yet to drive the XFR-S, but I reckon it should be pretty good as a big, slidey, shouty saloon.
The Integra just disappoints me. It seems half-arsed as an inclusion when they have other choices from the 360 one that could've made it over (Lotus Carlton mainly). And its an Acura, which still makes no sense for the game's setting. The only upside is that since they didn't chose to model a Euro-spec ITR, we don't have to look at the ugly rear fog light those models got: