Treo is a better product, I will grant you that. However it costs about $500-600 compared to $200 for a Sidekick. Size of the phones are fairly close, but since the Sidekick keyboard is built in below the screen, the keyboard is much bigger and easier to use. My buddy has a Treo and I have used it on occasion, it is annoying in that you could typo quite a bit becuase the keys are so small and so close together. Also the cost of service is much more for the Treo. With a Sidekick you can get a service plan for $30 a month, a reasonable amount of minutes with Sprint on a Treo or other providers (I think one or two others offer it) will cost at least $50 because you use your minutes on other phones while data usage on the Sidekick isn't measured.
In all if you really want a Palm device and plan on using the phone a lot, then the Treo might work for you. If you want something cheaper and just mostly to surf the web and keep in touch with email then the Sidekick is a great toy and won't bust your budget.