I joined on 3/31 and I love it! (I heard of it on here too, somewhere).
I'll look for that sweat/swimsuit challenge.
I've logged my food electronically for 7 years. I used Dietpower, which I still do for calculations, e.g. you can't type 1/3 cup into sparkpeople, it has to be a decimal expression, and dietpower always lists the weight per cup, which is great for a Brit like me who has scales and indeed cups but can't work out how to get a cauliflower into a cup lol.
The Best best thing about sparkpeople for me was when I joined, I was stalling seriously, and wondering, despite advice here, about lowering my calories. When I put my stats into sparkpeople, it told me I HAD to eat up to 1550 to lose, quite a different way of phrasing it that 'you can ONLY eat 1550 calories'. In my head, anyway. So I did and I have continued losing. 10lbs in 28 days.