You have to use the headset if you want to listen to the FM radio as it contains the antenna. Nokia has missed a trick (or saved some cash) by bundling a one piece headset with a 2.5mm connector to the phone. Had Nokia gone for a two-piece with a 3.5mm connector beyond the handsfree mike, you could have substituted a superior headset and still been able to listen to the radio.
I'm not over the moon about the buttons either. Beneath the screen the navigation pad is a raised frame around a central select key, and is itself framed by the two softmenus and Call and End keys. None of these is particularly easy to use. Partly I suspect the issue is that the size of the phone constrains the space available. Nokia has made an effort by placing raised lips round the outer keys, but still I found them a little pesky to hit accurately.
It also takes a bit of time to get used to the fact that the softkeys are not at the extreme left and right edges of the front fascia and so are not directly beneath the softmenus they map to. These spots are reserved for the Cancel and Nokia Menu keys, and it took me a few days to get used to this and stop hitting those keys by mistake.
The numberpad keys are rather easier to press accurately, though again the phone's size means they are a little small.
These things matter as whatever is going on inside a mobile phone, it is pretty much a waste if you can't use it properly. I think the 6120 is passable on the usability front, but that more could have been done with respect to the non-numberpad keys.
The 35MB of built in memory is not vast, but a microSD card slot sits on the left edge of the casing. It is protected by a solid hinged cover that is a bit fiddly to raise with a fingernail. If you lack nails I reckon you are going to have a very hard time with it. Populate a card with music tracks and the S60 player plays through its loudspeaker to an acceptable level.
The provided headphones deliver reasonable quality too. An equaliser has some presets (classic, jazz, pop, rock), and you can fiddle with left/right stereo balance, reverb (alley, bathroom, underwater, small room, medium room, large room, large hall), and set bass boost and stereo widening. These do have some discernable effect on what you hear.