For quite a while I couldn't connect to the internet using bluetooth /dev/rfcomm0 with the Samsung Z720, despite the /dev/ttyACM0 worked with with the supplied usb cable. The trick is to change channel 1 to channel 4 in the /etc/bluetooth/rfcomm.conf file (this is with Gentoo after unmasking the latest bluez-libs-3.28, bluez-utils-3.28-r1 packages and kernel is 2.6.24, but it should work the same for any recent distro). I didn't know the modem channel was different from 1... Now even both kmobiletools and gammu works using /dev/rfcomm0 like with ttyACM0.

The ping latency I get with bluetooth to reach my asterisk server at home (including the latency of the adsl, vpn etc..) is slightly above 100msec but usually below 150msec with HSDPA. With EDGE I usually get more than 300msec. Usb cable or bluetooth don't seem to make a lot of difference for the latency.

I didn't bother to measure the bandwidth with rfcomm but it should decrease it from 7mbit of HSDPA over ttyACM0 to 2.1mbit of EDR.

As for the AT commands needed to establish the ppp connection with HSDPA (or UMTS or EDGE or GPRS) this is my wvdial.conf:

[Dialer Defaults]
Init1 = ATZ
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
ISDN = 0

[Dialer z720-bt]
Init4 = AT+CGDCONT=1,"IP","your.apn"
Modem Type = USB Modem
;Modem = /dev/ttyACM0
Modem = /dev/rfcomm0
Baud = 921600
Phone = *99**1*1#
Stupid Mode = on
Username = yourusername
Password = yourpassword
;Auto DNS = on
Auto Reconnect = off
Dial Attempts = 1
Carrier Check = no

You likely want to turn Auto DNS on if you don't use a dns server behind the vpn. Hope this helps if you had my same rfcomm problem with the the Z720.