Wow, never realized kettle size was such a hot topic. Here are my real life experiences. I use
this aluminimum 40qt pot for my boils. It's about the cheapest 10 gal pot of decent quality your going to find. I've been using it for 2 years without a site glass, built-in thermometer, or ball valve just fine. I've made award winning beers with it.
In this pot I need to start my boil with 7 gal for a 5 gal batch. At a decent rolling boil I need to watch for boil-overs at the beginning of the boil and every time I add hops. I will inevitable have at least one boil-over. You can do a slow boil in a smaller pot, but that is a whole 'nother debate for a different thread. There are two serious disadvantages in addition to boil-overs to a 10 gal pot: 1. You can't do a 10 gal batch. 2. You can't do a 2+ hour boil (and end with a 5 gal batch).
Before you say I'll never do any of these, chances are if you stick to this great hobby you will eventually encounter a situation where you will want a bigger pot.
Now, if I were to do it today, I would absolutely, no questions asked, save for an extra month, and get a 15 gallon pot.
Here is one I've had my eye on. The extra capacity will be very useful in catching boil overs before they go over the side and allow me to do the occasional 10gal batch or two hour boil. Hope this helps.