I would like to do some watershed delineation at national/regional level (using SRTM-90 data).

I have experimented with r.watershed (GRASS 6.4.3) but it fell over when I tried to do big stuff.

I have tried r.terraflow (GRASS 6.4.3) but don't really know what I am doing with this... Seems quite old so maybe there is something better these days?

No luck with ArcGIS at all for big stuff (but available to me if somebody has a solution using this software).

Can anybody recommend practical tools for big watershed delineation projects?