Hi Viktor,
I got a Serenata at Christmas, and have been very happy with it since. I did have one problem with it to do with transferring music from my MAC, where the Serenata developed a problem, but this was resolved with my dealer swiftly. The speaker if fine for a bit of background music, but I have other things I'd rather listen to music out loud on when I'm at home or at uni. But when listening with the Earset 3 the Serenata excels, trumping my ipod classic and shure earphones. I don't have a BS2 or BS6 to compare it to I'm afraid. Texting can be a bit of a struggle, but I'm not a prolific texter and can get by. All in all though, very happy with my Serenata.
I did try putting some videos on, you'll need to make sure they are at the correct resolution and aspect ratio (mine were 16:9 and came up letterboxed), but its not a feature I use at all now. I dont think there's a file size limit, other than you cant exceed the 3.8GB available for media on the hard drive.
With respect to a keypad during a call, I just tried it out there and couldn't seem to get one to show up, either by touching the touch screen or pushing the buttons on the click wheel. Definitely something to clarify with your dealer before you purchase.
I have no experience of trading in products, although I see no reason for your dealer not to offer to trade in your products. If not you could always try selling them in the "For Sale" section here on Beoworld, or on an auction site(ebay).
Hope this is of help, and if you have any other question's specific to Serenata just ask.
Robert.