Any car with turbos has some kind of lag off the line. I'm used to driving naturally aspirated engines with great throttle response and I really feel any lag when it's there.
Manufacturers have gotten much better over the years making it less and less noticeable. If this were a real issue, BMW could add a small electric motor to spin the turbos for a better launch but it's just not an issue in practice.
Keep in mind on the 50i that even without the turbos, this is a great 4.4 liter V8. I think the only reason it's noticeable is that things get really crazy when the turbos spool up.
I view it as a completely appropriate compromise - you get the increase in power/torque and better mileage with a little lag off the line.
As usual, test drive before you buy.