i recently got a stereo set of MKH8040s and i absolutely love 'em. i'm now yearning for a set of multi-pattern mics to add breadth and options to my bag 'o' trix. after pouring over a lot of info posted online i keep coming back to the MKH800s. they are quite pricey, but they pretty much cover all the bases and would offer a lot of different solutions for a multitude of recording situations. i also like that the MKH800s have a high end boost, which is helpful when mics are placed a good distance from the sound source.
I own both the MKH 8040 and the MKH 800. If you like the 8040 you will like the 800 because they sound very much alike. Just to be clear, when you say the MKH 800 have a "high end boost" you mean the have a
switchable treble emphasis? Because without the treble emphasis tuned on the mic is quite flat across the spectrum. I have no problem with however Sennheiser achieves this response.
I use my MKH 800 most of the time as a side mic in a MS or double MS setup, so I have the most experience with its figure-8 setting. As a figure-8 it works just fine and matches the sound of the MKH 8040 or MKH 800 Twin Mid mic very well as would be expected. For my application perhaps an MKH 8030 would work even better but Sennheiser does not yet make that model.
I suspect the MKH 800 is a complex mic, but I have never had a problem with mine. They are expensive. When you find one used (not very often) they sell in the range of $1500 to $2000.
Although I would not use the MKH 800 in particular for nature recording (since it's expensive and many of its features are not needed in that application) the MKH series is probably the most commonly use mics for recording in humid environments. I have read many reports of various MKH mics being used without fail under very difficult conditions.