Please make a list of

Must have (essentials)
Need to have (the middle ground)
Can live without but like (expendables)

and set a measure after all those list-items are verified to evaluate if it is a good phone/micro-computer for you

Keep in mind that this device is a computer with phone functionality and not a smartphone in terms of calendar/address-book/phone-features. They are all kind of upgradeable by community tools but remain limited yet.

If you like to start your experience drive for linux with this phone you should keep your 5800 just in case you brick your device for some days and got no time to read all the forum posts to fix the problem.

If you decide to join us as n900 owner I would recommend to read some wiki pages and some forum posts about tweaks, must-have-apps and other things before you start actually using it as primary device.

The day you tell "I find my way around!" is the day to sell your 5800.

Hope that helped a bit