i just installed my aFe stage 2 intake on my N54 last night. by myself, it took me about an hour, but i also wasn't rushing.
not sure how much different the N52 model is, but for me, the hardest part was getting the damn rubber grommets from the OEM air-box to seat properly in the aFe housing. took a good bit of finagling and a lot of curse words.
other than that, everything was pretty straight-forward, so i'd rate it around a 3. again i'm not sure if the N52 one is much different.
'08 monaco blue E90 335i 6MT. sport pkg. premium pkg. black leatherette. brushed aluminum trim. M-Tech shift knob/boot. M3 mats. matte black kidneys. carbon performance trunk spoiler. blacklines. MTEC angel eyes. JB4. aFe stage 2 intake. custom downpipe-back stainless exhaust with 4" tips. mechunik plate mount. 19x9/19x10 +24 style 95's with 10mm BMS spacers in the rear.