"they make widgets with songs submited by artists, and that widgets contain ads. That means that a blogger, for instance, can have a music player with songs of his choice in its website, and the revenue from the ads in there are splited between the blogger, the artist of the tracks presented there, and Poptopus, of course"
The service is actualy quite good for what they aim to do, even if they still have some technical issues, and some stuff should be added, like ways to change the widget style for a better integration with your blog (see screenshot of the widget).
All in all, I think that, without being a killer service, Poptopus is preety cool for its niche market: if you already see people embeding in their blogs music widgets, I only see advantages for them to use Poptopus instead of any other, specially because they can choose the music they want to air, do their own playlists, and there are few music playback widgets as usable as this one. For those bloggers, this would be the best choice I know of, even if just to use it as it is now - without getting money out of it. But then you use it and can even make money from it... Isn't it just great? For musicians this is preety cool too. You can stream the music you want, in mp3 format with the quality ratio you want (they aren't re-encoded), people will spread the word about your music, and if someone points to a track they'll see an image of the album cover and, if they click, they'll go to the page of your choice (your official website, or an online store... whatever you want!). Once again, here I can only see advantages even forgeting the fact that you'll end getting paid for it!
PS -> If you're pondering on using Poptopus' widget, please consider playing one of Merankorii's tracks (Merankorii being my own musical project) ;-)