Performance

Some benchmarks to show systrace overhead:
Command                                Real    User  System
find /usr/src/ >/dev/null                30     0.2     0.3
systrace find /usr/src/ >/dev/null       42     1.2     3.8

gzip -9 test.bin                        2.0     1.7     0.1
systrace gzip -9 test.bin               1.9     1.6     0.1

Results	  
Find has a high overhead because of many open calls.
About 57k files in that subdirectory.
Gzip has no noticeable overhead because it is mostly CPU bound.
10 Mbyte file was compressed.

No noticeable end-user difference for desktop applications:
e.g. konq-e, mplayer or gaim.