I used to use a Rode NT4, which is a great mic, especially for the price. Like Nathan said, it is heavy. I really haven't used it much in years, but I seem to remember the stereo field is not as wide as some, and of course, it is not adjustable (the capsules are fixed in place).
Now I have a few different setups. I often use my Neumann 191 stereo shotgun. It is one of those great "what you hear, is what you get" mics. Also, very convienient to use a single mic setup. The only issues with the Neumann are that it is a little noisey for the quiet stuff, and being a shotgun, it can really pull things in close, which you don't always want for ambience recording. I generally run the Neumann in XY (you can select on the matrix box XY or MS). I've also heard there can be issues in high-humidity (like with Schoeps). I've stuck the Neumann out in the middle of rain with no issues, so, I don't know.
I also have a couple of Sennheiser MKH setups. I have 2 MKH800s that are multipattern. Depending on how I need to set up, I can run XY, MS, dual omni, or anything else. If I need to hand-hold, I'll use on of the 800's as a side and run an MKH40 as a mid. The 800's are side address, so they can be a pain when hand-held. When I'm doing MS, I tend to decode in post versus during the recording. MKH mics are amazingly quiet. Probably the lowest self noise out there. They also pick up an insane amount of low end, so you'll want to watch that when recording and maybe use low cuts when doing ambient stuff. I think the MKH series are the "dream team" of ambience mics.
Now since I've gone and wrote a book about mics, I'll try an address the rest of you question :)
When I'm recording ambiences, I generally try to setup and record for at least five mintues without moving the mics. I will often try a few different perspectives in the same area (facing different directions, moving 10 feet away, etc) to give me options, it really depends on how much time I have. I'll move the mics around listening, until I find the spots that sound the best. The real key is to record as long as you can without touching the mics, moving the mics, moving yourself, breathing near the mics, etc. I use the stopwatch on my iPhone and read through Twitter/Facebook while I'm recording :) And never adjust levels once you hit record, or the take is trash.