Here is a compilation of observations made by N900 users shared in this thread when using swappiness = 30.

Battery-life improved & responsiveness improved & no regressions
pycage
hackfanatic

Responsiveness improved
slender note: and page-cluster 0
JaseP
iareraccoon note: using knownokia.ca's settings compared to merely swapiness = 15

No improvements & no regressions
memoryguy
zlatko
jnack95

Note that nobody has specified which kernel they're using. I'm not sure whether this is relevant.

You're encouraged to verify my compilation, improve details on your (future) reports, and use above compilation as basis in next analysis of this tweak whether in this thread or on wiki.