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If you need a flexible Router and good Wireless then there is No good Combo Modem/Router.
Combo Modem/Router
advantages.
1. If your ISP does not give you a Modem buying a combo instead of standalone Modem and stand alone Router by buying a combo you night save few $$$ (at best).
2. Combo is comprised of Only one colored plastic box, if one has an allergy to plastic boxes it is a good idea to buy combo.
Disadvantages.
1. The Router, and the Wireless parts of the combos are mediocre and bellow (there is No combos among the high rated Wireless).
2. If there is any new device coming out and you want to upgrade the Router, or the Wireless, or the Modem, you have to ditch the whole thing (look around in forums, you would find people that bought DSL combos and after few monthes switched to Cable Internet. They get very surprise when they find out that they do not have a router any more:shocked

.
So why there is Combos.
1. Many End-Users really do not care about "Speed". Mediocre performance is good enough for their needs, and an extra plastic box is challenging their emotional status about the Décor.
2. It really came out as a need of the DSL industry that vastly employs PPPOE authentication.
In order to connect the ISPs used to provide PPPOE connection software. The software and the method used are quirky and generate many expensive support calls.
As a solution, many DSL ISPs started to provide Combo Modem/Routers (when they buy in balk it adds about $10 to the price of the modem).
Since Routers design includes PPPOE capacity in the firmware, the Router keeps the DSL connection a live in hardware, and there is No need for PPPPOE software. This reduces the number of support call, and balances the extra expense of the combo.