I wanted to remove the plenum to check the idle control valve's hose, which supposedly disintegrates at some point and the debris causes the valve to fail, but I didn't find that to be true: other than a 1" manual clamp to connect to the plenum, it's a regular rubber hose attached at both ends with those pesky BMW clamps, so couldn't check anything. Plus there's still a lot of crap to remove to get to the valve and strips anyway, so my job ended there, other than cleaning everything and checking bolts (the ones securing both PCV valves were pretty loose in my car, so give them a nudge). I'd NOT accept a hose replacement as fix; insist on a new idle valve.
Here's the procedure to remove the plenum (installation is in reverse order):
1. Remove air filter cover (8 T25 screws that don't come off the lid -nice). Wiggle the sucker out. There're DIY for air filter replacements if you can't do it, but I wouldn't continue if you have difficulty removing that cover
2. Remove the 2 10mm bolts that secure the bottom of the air filter housing and just pull sucker out.
3. Dislodge passenger's side sparkplug cover by pushing out from the bottom at both ends. Once loose, you'll have to remove the 2 10mm bolts holding the coolant reservoir in place, push it to the front to dislodge it from the green grommet on the wheel well, and wiggle both (one with each hand) until you can remove the cover.
4. Remove driver's side sparkplug cover. It dislodges in the same manner as passenger side, but comes out much easier. Just dislodge fuel hose from the top, and wiggle it under the hose.
5. Remove a plastic hose connected to the small piece between plenum and air filter housing; it has a manual clamp so piece of cake.
6. Remove plenum connector by the oil cap.
7. Loosen up all 8 TB (throttle body) clamps with either a long screwdriver or a 6mm socket with extensions.
8. Push the plenum up to break it loose from the TBs.
9. Once loose (it's easier), you need to disconnect another TWO hoses UNDER the plenum, both with manual clamps as well: a small one is in the front, below the 'snake' going to the air filter, and the big one at the rear, on the driver's side, where the brake booster should be. And finally, you need to remove the driver's side PCV valve from the valve cover. It has 4 T30 screws. You need to wiggle it as you push out, since there're an O-ring on the cavity forward of the engine that will make it a little hard.
10. Now the plenum will come out.