Oh, and a 306 gti-6 that hasn't been lowered, cheap as and massive fun!
Definitely! Very underrated and a surprisingly livable car too on a day to day basis.
The Mini may be overrated but it does have plenty of stuff going for it (cheap and reliable parts supply in addition to a great chassis in addition to being reliable if it's not a neglected mess; Most other cars tend to compromise on one of the above IME).
Dolomites can be underrated, IMO they are not right from the off (steering and handling is very soft as standard IME, and as mentioned by Rob, scope for lowering them is almost non existant due to the already low ground clearance! Some of these flaws do make for a surprisingly great cruiser though going up and down the motorway, far moreso than they should for a car of their time (quiet, dare I say even refined!). I remember my first long drive behind the wheel of a Sprint and it was a great journey all round, still feel refreshed on the other side . Biggest issue with the Sprints IMO is that many of them are not setup correctly with regards to the engine. A good Sprint should shift well, but they can be needy things there! They really need (and do respond well) to an engine tune, very much like the old Porsche 944S and S2 Twin Cams which can feel numb to some.
Acclaim is a great shout. So much scope and potential about them!
Porsche 924S should be added to the ranks of underrated especially in the rare 924S Le Mans guise.
There is another reason why cars can seem underrated to us. Our preferences do not align with those of most people . Some of us like smooth operators of cars (say the Porsche 968), others prefer a sense of rawness about the cars (old school turbo cars seem to have this present .
Here is another underrated car. Citroen AX 1.5D. OK they may not be quick in a straight line but with some thought out suspension mods and a bottle of wine in the appropriate door card holder for good measure they are a great amount of fun. I really was surprised at how capable a chassis they are when my friend had one and did the above for not much cash (think of it as a Citroen GTD ). He still misses it (an old dear saw the end of that car ).