EDIT---------------
Unfortunately my mechanic friend is no longer taking new customers due to lack of availability.
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As everyone knows, the N54 intake valves get crapped up with carbon after just a few thousand miles. By 50k, they are restricting intake airflow significantly. It's not as bad as Audi which has a very serious issue (up to 100 hp lost in 100k miles), but it's bad.
A friend of mine is an Audi Mechanic in central NJ but also has his own small shop at his house for weekend and night work. Because he works on Audi a lot, and the dealership he works at uses the BMW walnut blasting system, he is very familiar with the process and equipment. He charges around ~$45 an hour for jobs. I've been using him for suspension stuff that I can't do myself for a few years for my 135i and 535xi wagon.
Anyway, he got a walnut blaster with the BMW manifold system and is doing BMW carbon cleaning now for only $300. His work is awesome and he's located in Hunterdon County. If you need something else, he's likely cheaper than wherever you are taking your car now, too.
I would really recommend him for his work. His name is Phil. He asked me to make his contact information private, so please send me a PM for his information.
Before - 135i with 71000 miles, with meth injection
After
My 535xi looked far worse without meth injection-- so meth really does help.
As for a review, the 135i was misfiring slightly before which is now better. The same happened to my 535xi at 105,000 miles. The 1 feels maybe slightly faster, but with 465 rwhp from my last dyno at 63,000 miles, it's hard to notice. My 535xi feels much faster which has only a tune. I haven't noticed any significant fuel economy improvements, but I'm very happy with the service.