I bought 6131 for my wife's birthday last year and here are my thoughts:
1. Voice quality is great, typical Nokia.
2. Built quality is so-so. It's very plasticy and feels somewhat like a toy. The rubberized back of the phone will start peeling after few months.
3. Battery life is somehat poor due to a QVGA 16mil color display, so expect to charge your phone religiously, every day. It's a trade-off for having a beautiful screen.
4. Since 6131 was never meant to be sold US, do not expect any warranty service from Nokia. On my wife's phone the external display sometimes goes blank and will not comeback until rebooted.
I would consider two alternatives: 6133, which is T-mobile's version of the phone, or 6126, which is Cingular's version sans FM radio.