Interesting post Tighty and I'd certainly see Northants as great value but how about considering the following:-
Which county, as well as Surrey, has also made finals day every year of the competition?
Which county has won more Twenty20 games than any other?
Which county has topped their group once again this year to earn a third consecutive home quarter final?
Which team's net run rate was best in the country in the group stages?
See where I'm going with this?

Yep it's that equally "unheralded, unfashionable and great value"........... Leicestershire!!

Whilst I'd agree that usually 20-20 is about big hitters and very little home advantage, in the case of Leics I'd say the exact opposite is true.
Yes they've had some batting "big guns" over the past four years, such as Darren Maddy and Brad Hodge, but in the main their success has come about by batting first, posting an ok score, and then defending it like mad!
In eight group games only once have they conceded more than 143 runs which is testament to some tight bowling and very keen fielding. Indeed the county has five entries in the top twenty most economical bowlers of this years cup. The bowling is very strong in that the typical eleven that plays will contain seven regular bowlers which allows both variation plus some margin for error if one or two have an off day.
I'd also say that playing at Grace Road gives Leicester a considerable advantage too, both from some passionate support plus the fact that it's a big playing area and a historically low scoring ground which seems to catch opposition teams out.
You can get them at 7-1 on Laddies ( same as Northants! ) and 5/6 to win the qf both of which seem excellent bets. Incidentally you can also get evens on Northants winning at Notts which I'd say was equally a good bet.
So my tips for finals day are:-
Surrey, Essex, Leics and Northants