The strange thing is CHDK for Canon SX20 has never come out of beta. But I know people who use it without a problem. You might encounter few bugs here and there but it works pretty good apart from that. CHDK gives you lot of things which you would have loved to have originally with your Canon SX20 Camera. Things like Superfine Jpeg format, Raw files, Battery meter etc.
I know some are worried to use CHDK because CHDK allows setting up extreme Aperture and Shutter speeds and thinking this might damage the camera. But is not likely to harm your camera since the camera's mechanical limitations will prevent extreme values from being executed.
Here's the CHDK Installation Guide,
Remember you can't use CHDK with memory cards bigger than 4GB. So always install it into a 4GB SDHC memory card or lower. But if you plan to use Raw format then make sure you buy a fastest 4GB SDHC card in the market for this. Raw format files are pretty big so speed matters. Here are several recommendations,
- Lexar Professional Series 4 GB 133x Class 6 SDHC card ( 75$ ) - Little too expensive.
- SanDisk Extreme 4 GB SDHC Card 13$ - ( Better choice ) - Read Reviews