I've driven quite a lot of 40 series right from their launch. The first ones with the white roof and the black band (1992) around the bottom of the roof were basicly good tractors and a revolation compaired to the 10 series, but they had not got all (that should be any) of the bugs out of them. They got better each year. As has been said the blue roofed ones are the best and of those the New Holland badged ones with the rounded plastic rear fenders are the best of those, by miles.
The SL transmission had a few issues getting stuck in gear, but there was a mod' for them. The SLE transmissions have been pretty good. They can get a bit rough to shift, but a calibration sorts that out, there is a jump 2-3rd or 6-7th, (it's the same gear) 6-7th is rough on the road, the other shifts are fine.
There is more info on the dash than most tractors for the SLE models and the performance monitor is good. The 0-99 numbers for the poistion of the 3pt is a stroke of genius and I still don't know why all tractors don't use it. Once in a while it will "forget" it has a radar and the speedo will go bonkers. Just put the number back in to refresh it's memory or sometimes they just remember it again...
If it spits fan belts off it needs an idler pully. The air conditioning is very powerfull and will keep you cool as long as the rads' clear.
I've never known a problem with the engines, they do click for a few seconds from cold untill the oil is circulating, but are very reliable. There is room for 2 batteries, but there is only one. Fit 2 I say.
The swing back pickup hitch is more simple and stronger than those poxy Dromone things we all must have now, and have a better hook too.
The 7840 is a better tractor than the larger engined 8240, I don't know why, but they seem to pull better. The lastest 8340 you can find with blue roof and plastic fenders will make as good a tractor as you will ever need, but don't be shy with the front weights!