If you have a camera without onboard flash controls, e.g., Rebel 2000 or TI, you may use the the 220EX in automatic mode, but cannot access all the special features. If you have an older Type B camera, e.g., EOS 10S, A2, Elan or 1N, you can use the flash in automatic TTL mode only. Flash Exposure Compensation is available except with the EOS Elan/100 and Rebel series. However, most E-TTL features--FP Flash, FE Lock and second-curtain sync--are unavailable. See the compatibility table at the bottom of this page for more information.
The 220EX is surprisingly petite, about the size of a pack of cigarettes. It's so small you can comfortably carry it in a shirt or coat pocket. I power it with four AA lithiums to keep weight as low as possible. Despite its small size, the recycling time is amazingly fast, 0.1 to 4.5 seconds with alkaline AA batteries or 0.1 to 2.5 seconds with Ni-Cd AA batteries. That's faster than both the 420EX and the 550EX!
The shoe lock of the 220EX is especially fast and simple to use. Once mounted, you simply flip a small latch to lock it. Much more convenient than the dorky thumbwheel found on larger Speedlites.