It worth $3, and works well on the Iphone for sure. No other surf forecast app gives you a full 7 day forecast. Its usually pretty accurate. Personally for me its worth $3 to have an idea of swell forecast 5 days in advance, i have a pretty hectic shedule so if I can get a heads up that we might have some surf next wednesday I can start working my stuff around needing to get free that day.
All of the forecast data in the app is the same ad on the web site. So you can get all the forecast info through your mobile browser, but the app interface is much faster and easier to get the info. Eventually we will get a better interface for mobile browsers.
if your willing to miss half hour of work to surf, assuming you make $10/ hour, that session just costed you $5. if you live 5 min from the beach and check the waves 2x's per week, your probably paying $1-2/ week checking waves in gas. if the swellinfo app gets you 1 extra session per year, or avoids you from driving to the beach 5 times or so (total), you just made a great investment
as to alternatives....there are not many that work well for the east coast--mostly west coast focused apps...and to be honest, SI was the best. I bought it originally for my Itouch and then for my droid a year or two later. Only a slight difference with the apple app in that you can email the screen to friends or post it on a social media site...but still no love loss with the android app...such a convenient app...definitely worth the money!
Its the only app I have ever bought...just buy it...cheaper than a gallon of gas ;-)