Don't forget to factor in the additional $30 monthly cost of a data plan when you upgrade to a smartphone.
If you want to save money and switch to Sprint, just be certain it's the right fit for your needs. You certainly won't get better customer service w/ Sprint, nor will you get additional coverage, since they primarily roam on Verizon where they do not have native coverage. To be honest, mentioning an upgrade to a handset that has not been well tested by users (the Evo 4G) seems a little like the cart before the horse, especially if you already are satisfied with your current carrier. Switching just to get a particular handset is always a mixed bag. The only issue perhaps w/ ATT is whether or not they also have adequate coverage where you need it. This is the bottom line, really. 4G is mostly marketing fluff if you look at the speeds already present from the big two carriers (Verizon & ATT).
The iPhone certainly has one of the better user interfaces if you consider how nice web browsing is via the mobile version of Safari on the iPhone OS. With multi-tasking on the iPhone just around the corner along with increased screen resolution, perhaps that would be at least worth investigating provided you can still place a call where you need to. Since their technology uses SIM cards, ATT also gives you the opportunity to bring in your own unlocked/unbranded handset that would not require a mandatory $30 data plan. Keep on researching if you aren't in any particular hurry to upgrade...
Any suggestions for taking the plunge to a Smart Phone? I am out of contract with Verizon & not sure if the Incredible would be the way to go or switching carriers. Maybe iPhone or Sprint's new Evo 4G. It seems as soon as something comes out, something "better" is soon to follow. Any At&t or Sprint issues I need to be aware of? I have been very happy with Verizon and its coverage & service. Thanks for any advice.