Where to start?
On a normally aspirated engine, there's usually not much to be gained from a remap.
Smoothing out power delivery, optimisation for decent fuel (super unleaded) etc, and a few other things are all good, but you'll not see big power gains.
However, on the M52's, for emissions/tax band reasons, the engines were restricted to a lower than optimal output. Basically, BMW used a restricted inlet manifold to reduce the amount of air flow possible and then they were mapped accordingly to make the best use of that available volume of air. Think of it like only being allowed 80% throttle and everything else set up around that restriction.
The earlier M50 (E36 325i) had far better air flow which is why swapping the manifolds makes such a difference (+20 bhp) , but the standard map can't then make best use of this as it won't inject enough fuel at full throttle and looses torque in the lower rev range. That's were the remap really comes in and should release another 20 bhp and restore some of the lost torque. I also lifted my rev limiter to 7500, which is maybe a little high, but I'll prob never take it there when driving anyway.
I've always intended to fit an M50 manifold, and have one sitting in my garage along with all the parts to fit it (including the Orion version of my air filter setup), but haven't had the time recently to get round to the task.
I never actually drove my 2.8 Z3 on the standard map (car was dead when I bought it and needed an ECU) but I understand what we did to it and why, however, it was an off the shelf map that was used (from an experienced tuner) as I needed to get the car running and home. I've done some of my other cars myself with great results and piece of mind knowing that the engines not being pushed too far.
As for using some of the downloadable maps, I've no idea, and personally wouldn't use anything I didn't understand, but there's some great programs for analysis, like TunerPro and RomRaider, so if you are looking to tweak things yourself, download a few and see what people are doing differently and figure out what works for you.