over the yeas i became used to the following features that i expect a phone to have:

text2speech (both to announce the caller name and to read sms/mails to me)
voice dialling
MMS
GSM video calls
contact groups and ringtones per contact
a few java applications i use regularly

in pretty much this order.

With my main SIM-card in the N900 first, it was a strange feeling for me to hear the phone ring on the table behind me and not knowing who's calling. I didn't want that. I also didn't want to go through a phonebook to find a contact. I just press a button and say his/her name. And of course, having all my MMS-messages disappear into nirvana was not an option. I would not explain to my friends that no, I didn't see the picture they sent because my €600 phone doesn't have such advanced features as MMS.

So my SIM card went back into my old phone, the N900 has the secondary SIM now and things are back to normal. My operator routes incoming calls to both devices, and when I do make a call from my N900, the other person will see my main number, so I can still pretty much test everything on the N900 if I want to. But for every day use? No.

(Also, compared to my S60 phone, Maemo simply lacks applications.)