Follow what you eat!
I haven’t used those specific programs to solve those problems, but it takes a lot to replace your brain with one. If you are creating some other RSS player or Twitter client, in fact there are many other people who can simply use it, however, you will have to compete with well established and very mature programs. Competition in these categories is not a price query: it is a relevance and attention query. If you can’t locate enough consumers here, it’s probably not because it rates $2.99 instead of $1.99 or $0, but because its application doesn’t convince enough other people that it’s worth using compared to the alternatives.