I worked at a wireless ISP that serviced roughly 200 customers that were completely unreachable by traditional means. The location was set in the mild to medium forested areas of East Texas. We had a 30Mb pipe that worked quite well for our network we never saw it start to "peak" or be overtaxed. Being that we were on the 900Mhz spectrum, the fastest anyone could run at was 1.5Mb/s - 2Mb/s.
Here area some of my thoughts regarding setting up your own ISP.
1) It is completely doable. However, there are two roads to take. You can do it on the cheap, or you can do it the way that will stand time. My company chose the method that stood throughout time. What I mean is, we were not using off-the-shelf radios. We rolled out the network using the 900Mhz Motorola Canopy equipment. We used outdoor rated cable that had separation of twisted pairs and grease filled interior to prevent water issues.
Our main competitor, who worked on the north and west side of the city went the opposite route. He chose to use cheaper 2.4GHZ equipment, primarily PTP bridges.
2) The technology is out there, you just have to find it at a price that you are willing to pay. When I was servicing the radios, they would cost roughly $350 new from Motorola just for the endpoint Subscriber Module. We instead purchased refurbished models for almost half the price at $200-225. The Access points and other major equipment will set you back, IT IS NOT CHEAP.
3) Backbone and network structure. We may have over engineered our network, but we felt it was necessary to keep subscriber information private. We had a small cisco switch that at each access tower that would assign VLAN to each subscriber module. On the internal side of the switch, the VLANs were removed and went into a bulk VLAN that was specified for that tower. No other subscriber could see any other one without first going to "The Internet". We also created a Management VLAN, so we could service and access the management interfaces on each of the Backhauls and APs. Latency across the network averaged about 50-150ms.
4) Please for the love of all that is holy, do not, run your own Email server. It is a absolute pain in the ass. I was the person who was in charge of ensuring that the systems in place stayed running. This meant, DHCP, DNS, HTTP, Email Services, and Management interfaces.
Remember Virtual Machines are your friend. Buy one or two hefty servers and backup the VMs to each other. That way if you have an outtage, you can get the VMs back up in running in about an hour.
DHCP - Since we had a bit of a robust network, we had different subnets for each of our towers. In total we had about 18 subnets that each had different purposes. This tool helped like the charm that it was. http://phpdhcpadmin.sourceforge.net/ At the time the logout system was broken, however, I patched the code to disable the login/logout functions and wrote a script that would automatically give me the next available IP address.
DNS - No fancy tools here, I mostly just let it roll and didn't touch it. I only touched DHCP when we added a hosted website.(which later went to rackspace)
HTTP - Simple, run Apache, set and forget.
Email Services - Complete Pain In The Ass. No really, I'm not joking. At the time, the powers over me, decided that we would give our customers up to 5 email addresses. So I setup a linux server in that ran Postfix, Dovecot, ClamAV, Squirrel Mail. It provided IMAP, POP, SMTP and SSL(if wanted). At the time, when I arrived the server was already in place and running. However, fast forward, 3 months, and someone decided to run "updates" on the server. Breaks all of the packages, settings, the whole shebang. Not a fun week at all.
Besides that, there were also issue with SPAM. We would constantly get blacklisted by various servers.
Management Interfaces - This was where the heart of out network lay. I have one word, Cacti, http://cacti.net/ For wireless this was a God-send. As you could get information about the RSSI, Jitter, Signal Strenth, ETC about each device all nice and neatly laid out for you. I am the guy who developed the Google Maps plugin for Cacti. http://forums.cacti.net/viewtopic.php?f=19&t=33716 At the time there was no way to visualize Up/Down statuses and other things across a real map. This was the thing that really helped boost our ability to manage the network.
4) Would I do it again, Absolutely. It is great fun managing your own network, knowing that any problem you encounter is probably a stupid mistake you personally made. It a great way to sharpen your teeth and grow technologically in a new direction.
All In All, It Is Doable, It Is Hard, It Can Be Done.