It's a good small symbian smartphone which I have been using more than the N95 because of its small size. If I didn't have the Xseries gold dataplan on 3 then I would be using the N95 with its wifi more often.
Instead of the standard web browser give
opera mini a whirl, free but no https support, connections will be unsecure but it does strip a lot of unnecessary rubbish to reduce bandwidth usage, or theres
Opera Mobile. At the moment opera mini beta 4 supports landscape mode for browsing.
One problem I found when I first got the 6120 was the dodgy placement of the soft keys below the screen. After using the N95, I was used to the softkeys being on the far edges of the phone, with the 6120 these keys are very close to the central 'navi key', You soon get used to this. Also within close proximity is the hangup button, which on series 60 phones is a shortcut back to the standby screen. Pressing this by accident can cause some applications to close altogether rather than remaining in the background.
The navi-key is a bit thin so after a lot of pressing you may end up with a notch in your thumb.
The obvious downside after coming from something like the N95 would be the shortage of storage on the phone itsefl for installing applications. Non necessary stuff can be stored on the removable micro-sd card and installed as needed.
There isn't a SIP client if you want to use Truphone with a dataplan unfortunately so I may go back to the N95 for that.
The screen may be small but that goes hand-in-hand with the small handset size and is still QVGA resolution. If you rely on the clock on a phone when it has gone into sleep mode and the screen has powered down, then you might want to give
cClock a go. It displays the time in a large size as well as providing large icons for missed calles/messages and isn't overridden by the system powersaver, something than happened with other large format clocks that I tried.
Despite some small problems, it's a great little phone and with a decent data plan it becomes an essential utility. Currently I have Fring and 3's own Skype, Opera Mini and Opera Mobile trial, Shozu, Nokia Maps, Google Maps, Sky Anytime, MSN Messenger, cClock and a wake on lan utility from jibble.org installed.
If you have any questions just ask
[edit] with reference to the speed, it is much faster through the menus than the N95, as if the 6120 was a non-symbian phone, this was with my N95 on fresh generic firmware.