There's a great interview with Barrichello on youtube (in portuguese), a sports show called sportv linha de chegada, I won't translate everything but here are some interesting facts:
1)The car started being developed in May 2008, and according to Rubens the aerodynamicists found very good solutions for the new rules quite early. He was glad the car was unveiled so late "so the other teams couldn't copy it".
2)The car is 20% more rigid than last year's version and when they were adapting the mercedes engine to the gearbox there was a 8 mm difference that caused some concern (he said they didnt want to lose chassis rigidity).
3)The new car was developed by the same people but under close supervision by Ross Brawn, he gave an example of aerodynamicists contacting Brawn and asking whether a certain change worth a few aero points was valid and then he would approve it or not).
4)With the honda engine he suffered with understeer at turn exits (he said he'd accelerate and the car would still understeer whereas with the mercedes engine he can correct the understeer with the engine), I don't know if this is useful to anyone, but I'll translate whatever I think is interesting...
5)He said ferrari didnt do quali simulations (he said even when he was there they didnt do it to avoid showing all they had), so the best way to compare all cars was race simulations. As the cars carry 90Kg of gas at most with 0.3 to 0.4 sec per 10Kg per lap time difference it's very easy for a bad car to show good results. Then he said brawn couldn't afford to waste time showing off because they needed the mileage, so the times we saw in barcelona and jerez were for real.
In his opinion Toyota, Williams, Ferrari are probably the fastest (he said toyota didnt show all their game, bmw were running light,renault could be a little behind, williams with rosberg seemed fast, mclaren seemed to have problems, even driving behind their car he could sense it).
6)When he was doing this evaluation he said "we never know"
but he said that mclaren problems were obvious and they'd have problems for the whole 1st half of the season. He said that in the beginning they were strong and then all other teams improved and they didn't.
7)About KERS, he said there's a rumor that renault only improved after they removed it and re-balanced the car. He said that depending on the results and sponsor money they could get in the first races they could consider using kers (since mercedes already has one). He said something that was already pointed out by someone on the forum that every car weighs 605Kg (665Kg with gas at starting line, approximately), so with or without kers all cars weigh the same. He said that when he worked at ferrari (meaning 3 or 4 years ago, not because of the team) weight distribution would take an hour, disassembling the car and moving parts around, but nowadays it takes 5 minutes. He said you can change around 2% of the weight forwards or rearwards, which completely changes the car's behavior.
He said the moving front wing will definitely improve overtaking, with 6 degrees of range and 2 presets on the steering wheel he can change for instance from -1.5 degrees to +3 degrees and then to 0 degree if he presses the button again. Kinda confusing but he said if you press the button again during the lap the wing will go to 0 degree (and not return to -1.5 degrees).
9)The reporter asked him if they were bluffing, Rubens said he'd be very ashamed to put on this kind of show and then going to Australia and starting from 10th position. He said he even checked how much fuel was loaded himself when he first raced the car and was so surprised by the speed.
10) He said this year he's very happy to be in a position to do something he couldn't when he was in ferrari or when honda had a slow car, which is to race to win, so he'll be "knife in mouth". This is the first time he said he doesnt have to follow team orders like in Ferrari, so he'll do himself some justice... let's see.
11)His salary and Button's was lowered for this year, his is at least 5 times lower. But he said that they have a bonus system that might give him what he used to earn. I think this at least shows how much the guy loves the sport...
I'm sorry if this was too long and maybe nothing new, but it's great to hear it from the pilot himself on a tv show that's relaxed and friendly where he seems to be speaking his mind.
http://www.youtube.com/user/F1Sportv