I'm a stevia convert. It is used a lot in other countries with no ill effects. Here is a site with info on studies that have been done:
http://www.stevia.net/safety.htm
It is better to start out with less than you think that you need and then add more if it isn't sweet enough. I get it in the little pre-measured packets and use about half a packet in a cup of coffee to equal a scant tea spoon of sugar.
Honestly, at the dose I'm using, I really don't notice any difference between it and sugar in taste.
I avoid Splenda when I can. I prefer to use Stevia instead. Stevia has been used for a long time in other countries (it is derived from an herb), but hasn't been approved over here yet as a substitute sweetener. You can buy it in healthfood stores and some pharmacies as a "nutritional supplement", though. It comes in several different forms - dried leaves, liquid extract, powder extract and powder extract cut with other ingredients and packaged into little single serving size packets (like they do for other sub. sweeteners.)
I find that I only use 1/2 packet per cup of coffee and that (to my taste) equals the 1 teaspoon of sugar that I was using in my coffee.
The only trouble with using stevia is in some recipes that require the bulk of sugar or a sugar sub. like Splenda. I use stevia in all other instances, though.
The trick is to start out with less and add more - it packs a sweet wallop! If you use too much, you may get a licorishy aftertaste or bitterness. It is pricey (though splenda is, too) but you will be amazed at how little stevia you have to use.
You can also grow the herb in your garden - though it doesn't over winter well in zone 5 where I live - so I treat it as an annual. You can brew the leaves with tea and it tastes great.
I switched because I wanted something a little less chemistry lab that had a track record of safety.
It is not approved by the FDA here, but it is used extensively in Japan and Europe and has been for decades.
I believe that the Curves brand protein shakes are sweetened with stevia. Uh, I mean supplemented with stevia, (in case the FDA gods are watching.)
PS: quite a lot of controversy surrounds the whole stevia FDA issue - some folks suspect that Big Business doesn't want stevia to be approved as it would cut into the profits of other sweeteners...you can find more info on the internet if you are interested in more info.