The iSyncr app for your Android phone is a perfect solution for people who are tired of making room for both a phone and an MP3 player in their pockets. iSyncr allows users to sync up their iTunes playlists with their Android phone.
This app comes in both PC- and Mac-compatible versions, both for $2.99. There is also a lite version of both apps, but these restrict users to a single playlist download.
When the program is first opened, a pretty thorough tutorial explains how the app works. Getting yourself hooked up and syncing is, unfortunately, not a very intuitive process, but if you follow the directions, you should be able to easily figure it out.
I ran into problems when it was time to mount my phone's SD card. After I followed the instructions to connect to USB Mass Storage, I kept receiving a notification that the memory card inside of my Droid was missing. Because I clearly had photos and ringtones stored on my phone, I had to do a Google search to find out what this meant. I was able to resolve the issue by changing the USB connection to PC Mode before changing it back again to USB Mass Storage.
Spoiled by how quickly my iPod syncs up with my computer, I was very impatient while waiting for these files to sync with my phone. Thankfully, the phone didn't lock up during the data exchange, and I was able to make calls and text as usual.
The files were easy to find (just click on the Music shortcut within the iSyncr app), and played as smoothly as they would on an MP3 player.
One flaw with uploading music this way is that Android phones supposedly do not support audio files with Digital Rights Management. I did not have a problem, but it sounds like it's not guaranteed that your entire music library will play on your phone.