Depending on how strong you are and how much you enjoy pushing yourself, fixed gears are fine for at least moderate and rolling hills.
I have a basement full of bikes (hybrid, road, tandem, hardtail mountain) but my fixed gear is the bike that sees the most miles. I think it is a difficult thing to explain and it certainly won't be the favorite riding style for all people. I got started with Fixed gears after reading the writings of the late (well, he was still alive then) Sheldon Brown on the subject. He wasn't some hipster who wanted to earn street cred by doing brakeless skids in the mall parking garage. He was a serious bike enthusiast and mechanic and he did a better job than pretty much anyone else I've ever talked to at capturing the magic of fixed gear riding in words.
First and foremost is fixed riding pushes you to be a better rider. Maybe you love coasting and that is fine but to get stronger and better at riding you need to turn the cranks and a fixed gear is like a mean old nun at Catholic school wrapping you on the knuckles with a ruler if you try to get lazy with it. Some of us like that.
Second is the connection you feel to the bike. This is a much harder thing to explain but you feel more connected to the bike when you can start and slow/stop it without using controls. You take cycling from begin a body on top of a machine to the machine being almost like a prosthesis or an extension of you. I still have a brake on my bike because sometimes the hill is too steep or I need to stop quickly but in general I like to be able to "engine brake" the bike to moderate my speed.
Third is the fun factor. Because most off-the-shelf fixed gear frames are based on a track or track-ish geometry the bikes have short wheelbases and feel much twitchier. This twitchy steering gives sort of a fighter-jet feeling to the ride. Every little correction makes the bike bob and weave. This can add a certain excitement factor to your ride.
Yes, they are a lot more tiring to ride. I lived in Knoxville, TN in the foothills of the Smokey Mountains and put many hundreds of miles on my fixed gear all over the place. Most of my rides were 30 miles or less but even 30 miles of constant pedaling up and down hills is probably comparable to more like 50 miles on a geared/freewheeling bike.
My fixed bike as it looks today.
Before the latest paint job.
See also: http://sheldonbrown.com/fixed/