Well I've had the bold for 6 months and just changed to an 8900.
Loads of plus and minuses both ways, my only real reason for changing is the 8900 has uma and i have bad signal at work so routing calls over wifi has solved a big issue their for me having to call people back all the time cause i cant hear them, plus i do a bit of roaming and it will cut call costs using wifi hot spots to make calls.
Plus side of the 8900, smaller handset, sits much nicer in your hand, nice tactile feedback on the keys though keyboard is a bit small.
Plus side on the Bold is the keyboard is bigger, cracking screen for watching films (not tried the 8900 but expect it to be as good) it has 3.5g now whilst i didn't really think i would miss this their is one big BUT to that. I don't miss the data speed, i only ever use it for browsing forums when im not in wifi range and e-mail which 2.5g is plenty fast enough for but the one thing i never thought of is with 2g you cannot have an active data connection whilst a call comes in, so if your phone happens to be downloading an email or your surfing the net when someone rings they get bounced. This was not a big problem because 60% of the time im on wifi the real bummer for me is I use Garmin to navigate a hell of a lot, now that downloads maps whilst on the move and hence while its downloading and updating my journey my calls are bouncing, if I make a call the map stops updating after a few minutes.
I've still got the bold and as of yet I've not switched back to it and sticking with the curve but give it time and i might. Lucky i have Single number on Orange with two active sims on one number, meaning instead of leaving my spare phone in my bag i will very lightly have to switch it on whilst driving to pick up any calls I miss through using the Curve for Gamin navigation.
Unless 3g makes a difference to you I would go with the Curve, if like me it causes you issues then you wont be sorry with the Bold.
I know i waffled a bit but i hope it gives you some pointers in the right direction
