WiFi is great on the N900, and signal strength is as good as on a laptop.
With power-saving, a constant WiFi connection doesn't drain the battery much, unless you're constantly transferring data, of course.
The power-saving settings on the N900 are pretty high and you might experience problems with some WiFi routers (e.g. D-Link DIR 615), but setting the power-saving level from high to medium on the N900 did it for me. Even with medium setting a constant WiFi connection doesn't drain the battery much.