I can tell you that the N900 provides quite a bit less of always-on 100% CPU than the Palm T|X (torture test is in the 4 hours vicinity).
Of course, 100% CPU usage is not everyone's usual activity.

If you play any kind of emulators/games, it is, though. If you just read PDFs in offline mode I'm pretty sure it will handle around 6 hours. Also, unlike the Palm T|X, enabling Wi-Fi doesn't instantly kill the battery if the AP properly supports power saving features, and the great standby with Wi-Fi battery life is something I _really_ enjoyed when I made the jump from the Palm T|X to the N810 (whose battery life is unfortunately higher than the N900's).

Btw: There's nothing I know of anywhere near the quality and ease of use of RepliGo.
Though you'll find that reading "desktop", unreflowed PDF documents on the builtin PDF reader is acceptable*. Specially since the rendering speed is _miles_ ahead of anything PDF on the Palm T|X and even better than RepliGo's custom format. And the increased screen resolution.

*Though still not much usable for long ebooks. For my last ebook I used "Calibre" (a desktop e-book "manager" application) to convert from PDF to ePub, which I sent to FBReader on the N900. It worked nice, though it strips most images, fonts, etc.