Visa works directly with Banks --you need ATM to access cash. Mpesa succeded because of lack banking facilities for Majority of Kenyans. Will visa integrate with Agent's outlets that make Mpesa convenient? It will be good for Visa Card holders who can transfer amoung themselves and their banks. But..this are the minority and may not be Mpesa regular users.

This is my two cents worth of input:
1. Why is mpesa so popular?
2. Is it only popular to the poor unbanked and only to those with no access to "visa"?
The new provider will beat Mpesa if answers those questions and is able target the market from that angle.
1. to me(representing the average Kenyan) mpesa means, I can pay my power bill from the comfort of my sitting room I needen't waste precious time queing at KPLC offices, I can send money to my kid bro in Kenya Poly , the boy needn't come home every weekend,now I think my aged mun has forgotten how to scratch and upload airtime for her phone since I buy it for her via my phone. If I need something from town, i needn't drive all the way while my freind is working there, I
just send him the money pap and voila, I save on fuel and time. Who carries loads of money on their wallet any more while travelling? Jua kali people have a bank everywhere, meaning they can withdraw sell, and 'bank' without the limitations of time. the list is endless. To me that why its soo popular.
Convinience
2. Mpesa is no longer for the poor unbanked. Banks are taking advantage of it, companies like KPLC have a "business" number with mpesa and all others you know like insurances etc, hell you can even pay malipo ya uzee( NHIF) with mpesa thses days. Its simply an idea whose time is perfect here.
Let them join the fold by bringing in new ideas that will bring something new to the market.
My thoughs.

Yeah, typpo among many others.. I clicked post too soon...

WHERE ARE YOU NOW HATERS! IF I WERE AN AccessKenya Shareholders, i would be more concerned!

Well said sir! In Greek we say, 'Matariria kwa Shoo!'