Russell, since you have a BMW - I assume it has the OBC - try this for a while: measure your fuel mileage for each tank using the OBC, and also calculate it using gallons-per-mile-driven at each fill-up. On my car I have noticed that the difference between the two figures is a very good indication of injector cleanliness. That makes sense, since it seems reasonable that a dirty injector would deliver a bit less fuel than the computer
thinks it will be delivering.
When not using any cleaner in my car the difference between those two numbers drifts up to about 3% or a little more - about a 1mpg - 1.5mpg offset. I have never seen it consistently above that range.
Using a bottle of Regane or SI-1 will usually bring it right back down to a perfect match (varying between ~1% and ~-1% difference).
Recently I have been experimenting with smaller amounts of SI-1 in each tank. Currently I am using 1oz per fill-up, and it seems to be keeping the injectors in that -1% - 1% range.
I think Johnny is onto something, too. The SI-1 advertises itself as a top-end lube as well as a cleaner. I have seen that using a top-end lube will keep a light coating of oil on the valves, and it makes sense that it would help to prevent additional desposits from re-forming:
The above photo was taken when using 4oz/10gal MMO in my car - the recommended dose.
I am currently using 2oz MMO in each tank along with the SI-1 and believe it is helpful. The combination adds about $0.90 per fill-up. Not totally inconsequential, but less than the difference between top-tier and other fuel, for example. The SI-1 component is equivalent to using a whole bottle every 6k miles or so. I have little transparent travel-size shampoo bottles that I pre-fill with the right mix for each fill-up.
My car, if you don't recall, is an E30 325i, with the older two-valve M20 engine.