1st caveat: I am very cheap and never pay for anything that I don't feel like I need.
Is there enough demand for services like this? I have lived in very large urban areas (Toronto) where the public transportation (taxis included) are well used and well implemented. But I still see dozens of empty taxis (at all hours) queued up near theaters, bars, conerts and other similar venues waiting for clients. Empty/Idle taxis are losing money.
I always hear on the radio that taxi companies wanting the city to allow more licenses to be given out, so it appears that there is demand for these services. But alas, i always see a lot of them idle.
I have had to take a taxi 3 times in my life. All three times the fare was over $20. If I were to replace my bike and car for taxis and public transportation it would cost me more then $800 per month, which is almost as much as rent!