1. Is there anything wrong using a PDA as a MP3 player?
If this is some moral/ethical argument? :wink: ...no, the simple answer is no, the 2215 is a great MP3 player...if you're one of those who doesn't mind using flash-based players, and constantly updating songs, then yeah nothing wrong with that. I used to use my 4350 as my main MP3 player, and I have to tell you the sound quality was among the best I have heard for any audio device. As for the comparison with the ipod, the ipod is a load of rubbish: Apple have no idea how to make a real music player

. Look how crappy the sound is on their G4s!
Anyway, I found it annoying having to constantly update my playlist. There is one app which is essential if you're going to use your 2215 as an MP3 player: Playlist Sync found at pocketpcfreeware.com. It automatically updates the playlist for you.
But if you're really into music, than nothing still beats a dedicated MP3 player. I bought the iriver Ihp120: in my opinion a truly awesome product, you should check it out.
2. Using the HP iPaq 2215 as MP3 player, how good is the battery? Jason's review said "Display off, looping MP3 audio file: 8 hours 35 minutes". Can I really count on 8h of music?
You can't compare battery life to a dedicated music player (my iriver, for example, lasts 16 hours), but it is decent for most people. I am very suspicious of the review you quoted: '8hrs??'. My 4350, 1,500 mw battery lasts about 6 hr 30 mins looping a song, and since I think the 2210s battery is a 1,250 mw then it would last a shorter time. A more accurate estimate would be about 4.5 to 5 hours.
How about MIDI player? A search on Google shows that there is some MIDI player for Pocket PC. If you have any experience in this area, I'd appreciate some leads to more documentation.
I don't exactly what you mean by a MIDI player. If you mean a software app that plays MIDI files or WAV. MIDIs than Winamp should be fine.