I've seen a number of people complaining about slow or "bad" DVD playback on Sam460ex machines; usually, with some muttering about how missing overlay support in the Radeon HD drivers is ruining everything. Well, I strongly recommend that you check if your DVD drive is using DMA. Playback on my system is pretty good. It drops the odd frame, but it is relatively smooth.
The first step is to check what mode the drive is doing, and that can be done using idetool, e.g.:
idetool -l sii3114ide.device
Replace sii3114ide.device with the driver for the SATA/IDE controller that your drive is connected to. This will list something like Delta got, here. The most important line is "Xfer mode." If it says something like:
Flags : $0000011D - present, supports DMA, removable media, ATAPI, interrupts used,
Xfer mode : best pio 12 (PIO 4, 16 MB/s) / best dma 69 (UDMA 5, 100 MB/s) / current 12 (PIO 4, 16 MB/s)
then your DVD playback problem is because your drive is in PIO mode. Check what the appropriate *_xfer env-var (e.g., sii3114ide_xfer for sii3114 based SATA cards) is set to in UBoot. If it is not set, then try setting it to 0000. You'll find more details in SYS:Documentation/IDE/sii3114ide_dev.doc. That will set your drives to use the best transfer mode available. Alternatively, you can set the env-var to directly set the transfer mode to the maximum speed listed by idetool. The details for this can be found in the documentation.
Please note that this doesn't guarantee smooth playback. I once installed a new DVD drive into my A1-XE, and this resulted in very poor DVD playback. A depressing number of DVD drives aren't up to standard, and I happened to have bought one of the poor ones. Ripping the DVD to hard-drive, and playing it back from there resulted in perfectly smooth playback. If you find that playback from DVDs is poor, but the same video played from the hard-drive is fine (and both are using DMA), then your drive is one of the poor ones. In some cases firmware updates released by the drive manufacturer fix the problems, so that's worth checking for if you have such a drive (NOTE: you'll need a PC with Windows in order to apply the firmware update).