I've got Investment Detective and love it.
All I have to do is plug in the raw data, everything from purchase price to insurance costs and it tells me how much I will make (I like to put the optimistic first) or how little it will cost me per week/month/annum.
I can also change the fields on a "what if" scenario, so that I can see what happens to the overall performance of the property if I can get it cheaper, or up the rent or whatever.....all with a couple of simple clicks.. no calculator needed (thank god, math was never my strength)
Costs about 200-250 from memory. I wouldnt be without it.
Luvvit.