I used to use the TrAAm, and had varying results. In general, I try to avoid such things whenever possible- even at places like LHR where they have buses. When I'm on my own two feet, I know exactly what to expect, and I'm not subject to schedules, breakdowns, etc. Besides, walking is far better for one than riding. Of course, if the layover is really tight, one may be at the mercy of the transportation system. Personal philosophy- walk whenever possible.

Cheers.