I came from iPod classic 80GB too, until my wife "stole" it so Im left with omnia as mp3 player B) . And I was using CX500 too with my iPod, but had to convert to other earphones that can be disconnected at the middle, using CX500 is too long for me.
1. If you are using WMP, you dont have to change player if your problem is just the small control, you can change the skin that have larger controls, I have attached a skin here you can try, you can look for other skins if this doesnt suit you.
2. Had same problem, I had to choose which music or albums I usually play the most. After creating a playlist on itune (smart or simple), I select all (CTRL+A) and copy and paste to a folder on my omnia, like I would create a smartlist of my 5 star songs, select them all and copy and paste them to my omnia on a folder named "Favorites". same with album. On other songs, I had to rely on the folder, just copy and paste the folder. Luckily iTunes embed the artwork and other info on the mp3.
3. About the sound quality, I think its comparable to iPod, samsung isnt like other cheap cellphones that have inferior SQ. Just a comparison, my Omnia sound better than my PSP.
4. I once tried playing my omnia from home to work to home, from 100%, it took around 8 hours to drain the battery, but my mp3 is VBR 128 to 320.
after answering all your question, i kinda miss my ipod, i just dont have the balls to get it back from my wifey :( WMP_Skin_iPhone.cab