I too use my N900 daily.

I haven't found a suitable replacement yet
(Hardware Keyboard, Linux & Phone, small)

As a Phone it is perfectly usable.
SMS, phone, mail, chat everything works.
In my experience GPS on the N900 works way better than on the N9,
but also a fix takes way longer than with A-GPS on a modern phone.

Internet Browsing is difficult on some sites.
I am happily browsing all my daily news site (Javascript disabled)
And the only site I miss is gog.com, which brings my N900 to its knees and does not work without JS.

I love the ability to run Linux tools, to use ssh to check on our company server and to run java software.
Jbidwatcher for example is great. Nothing better as an ebay snipping platform than my always on pocket device.
Also there are so many file formats other phones cannot handle, the N900 opens nearly everything (with chroot).
On the other hand, I miss some android apps and would enjoy a modern game from time to time. Some of these can be replaced by websites.

My original battery is replaced by a Polarcell one which lasts 3 days without use and about 6 hours of heavy use (And I use it often, so I carry a spare)

What does the N900 lack:
Ram, modern websites and many applications which run under chroot want more than 256MB. Also compiling big software on the N900 itsself is impossible. This is the main reason I am looking for a replacement.
CPU is supprisingly ok
The camera was very good when it was released, but today it has low resolution and can't take videos in acceptable resolutions. Therefore I carry a 808 Pureview with me for these tasks.