Those prices are new I'm assuming? If you get one on eBay, new but second hand, the price can be very different.

A lot of the disappointment from the N900 comes with people not knowing how to change something, or not realising it didn't do something etc, but your second post made it very clear you fully understand the device, what it does and doesn't do, community support etc, so I see no reason for you not to get it.

As for lifespan, i think it can live for 2 years in terms of it not breaking, and you still have older phones so clearly you're ok with that prospect and having a device that isn't fully supported. The community is, as you said, incredible, I've never known anything like it and until i got this phone i wouldn't have even thought about, let alone dared to, customise it to such a level, but the community has allowed me to overclock, change icons, adjust themes and so forth. I assume the community will continue working on it for a while to come but obviously there's no guarantee, and we must be prepared that many N900 developers will go to MeeGo (but MeeGo apps should be portable to the N900). Without Nokia's support we won't get full updates like new firmware, but we may get additions and fixes like MuhammedAG has done.

If you want multitasking and a decent keyboard, N900 all the way!