Ooooo foooking heck I do.

My commute starts with a slight downhill run into a roundabout and then onto a major road. By 'slight', it doesn't look like a hell of a lot but going down hill, I'll be pulling 40km/hr in no time on the geared bike. Now, bearing in mind that I'm stone cold at this point and have an aversion to tearing muscles, that first km or so takes a bit of self control on the way out - on the way home when you're tired, it's just hard work. The next 5km are steady rolling hills that aren't a huge challenge in themselves except that once again, you haven't warmed up until well into it so you have to be careful not to try too hard going up the things or spin out too much going down. I usually feel properly warmed up and supple about half way along it. The last km of that stretch is a fairly solid uphill pull that fit riders on good bikes tend to work up without an issue but you do see a lot of lesser riders struggling upon it.
This brings you to a big roundabout that is always very busy and, thanks to its size, has two wide lanes you have to cross so, although it's flatish, is a challenge in itself. The road tends downwards for a bit, the ducks down enough to let the legs wind right up until a slight climb before starting the downhill run on Flagstaff Hill.
Flaggy Hill proper starts with a shortish downhill that is enough to wind you up to a decent cadence regardless of your gearing.
Then it drops down - it's an 8% downhill run for the next 2km though it eases at the bottom of this. On the geared bike, I take this at 70km/hr, on the fixed gear, I'm pulling and maintaining a cadence of 180, regardless of the gearing (the S3X gives me a top gear of over 90gi and I'm still pulling 180). There's a very short false flat and then you hit a 1km run that is 10% - a whisker less for half of it, then it kicks down to over 10% for the rest of it. You have to ride this in heavy traffic because it's a major road and there is no bike lane. The side of the road is pretty rough so it's better on the geared bike where you can stick with the cars and take the lane, on the fg, you can only spin like blazes and avoid the worst of the bumps. Immediately at the bottom of this run is a brief 100m to a set of traffic lights on a major road (4 lanes of always heavy traffic, only the dead run a red there) so it's off the bottom of the steep bit and hard on the brakes for a gasp at the lights. The next 2km are relatively straight forward - straight, sort of smooth, bike lane and steep enough to hold 40km/hr on the fg and 50 on the geared.

That is the first 9kms of my commute to work ... and, indeed, any ride that takes me towards the bulk of the city.

On the way home, I get to do it all in reverse. The bottom slope is steep enough to get the heart up and to take the sting out of your legs, then the first climb up Flaggy Hill just hits you in the face. I tend to walk a lot while leaning over the bars having a gasp makes sense some days.

It's an intense ride, even with gears and to be honest, doing Flagstaff Hill on a fixed gear is just stupid - they invented gears for a reason. But why should I forgo the joys of riding fixed for only a small part of my riding? I'd like to claim I'm a hard, highly tuned, cycling hero but apart from developing a good technique and the ability to hold stupid cadences, I'm just an average cyclist.

I run a gearing of 66gi on the fg. That works for me as I don't mind revving out a bit on the flat anyway.
My geared bike has a 30-52 triple on the front and 11-32 on the rear. This helps tremendously on that hill.

Going down hills is a matter of learning how to sit ON the saddle ie, your weight must be on the saddle, not your pedals. This allows you to spin your legs - if you're bouncing, you can guarantee you're putting pressure on your legs, probably by trying one of the ridiculous roadie pedaling techniques (notice how they can't spin past 110?).
Getting back up, I'm learning, is not a matter of putting ****loads of power onto the pedals. This is, in fact, counter productive. Similarly, if you're pulling and tugging at the pedals, you're wasting energy and spoiling your action. I do like to concentrate on the part of the stroke across the bottom of the circle but only because I find by doing so, I'm removing the tendency to push hard on the way down then pull up. Like a high cadence, the secret seems to be a smooth, fluid rotation of the pedals. You may have noticed that if you stop on a hill and have a rest, when you start up again, it seems stupidly easy and that's not all leg recovery, it's largely technique. Unfortunately, on a steep climb, sooner or later, it becomes difficult not to start forcing the pedals and I'm this is counter productive.

The alternative is to stand and climb. Grant Petersen talks about standing and only placing your body weight on the pedals. It all sounds so easy in his book. My experience has been that standing drives my heart rate upwards however, I'm beginning to wonder if I'm doing it wrong. Something to work on (always something to work on).

I do have an alternative to the Flagstaff Hill part of my route (only the one alternative outside of taking the car). That's longer, starts with a very long 8% climb followed by a few short climbs and ramps, then a very nasty and steep climb that peaks at over 10% only to ease into another long 8% climb. I refer to Flaggy Hill as the 'lazy man's route'.

I'm considering moving down to the flats ... but would do a lot better if I rode more often than I have been, lost a lot of weight, sorted out my technique ... and moved down to the flats.

There are no heroes on hills, just people fighting their own wars in their own fashion.