I currently have a 4+ year old iRiver H140 which is a hard drive based DAP (digital audio player). It has served it purpose playing OGG files and I am looking to buy a new DAP.
HDD based DAPs are nice because of large capacity, but they can also be a little bulky. The hard drives tend to develop problems over time because when you walk with your DAP, it will be swinging back and forth while the drive heads are trying to read data every now and then. Over time that can make HDD DAPs become a little unstable.
I'm looking at getting a flash based DAP. The good thing is no moving parts so you can even jog / run with it without having to worry about damaging it. Flash players also have longer battery life because of the lack of moving parts. The downside is that they are relatively expensive compared to a HDD based DAP with higher capacity.
Personally, I am currently looking at buying a Cowon D2 8GB which can be expanded by sticking in a HD-SD card. That's high density SD cards up to 32GB for a total of 40GB.
Cowon D2 8GB
The Cowon D2 has a reputation for outstanding sound quality from the time the first versions were released. The reason the D2 is high on my list is because of the 8GB capacity (16GB would be better), HD-SD card support, and the fact that it supports FLAC files (I'm not a fan of MP3). The ability to watch movies on the go is also a nice feature.