Single speed to multi-speed conversion with internal gear hub

If you find IGH that fits your rear fork, it is usually quite reliable, but somewhat pricey setup, since the cheapest IGH start around 100 Euros.

Here are instruction how you can measure your rear fork

you would need to find hub with same IGH measurment.

Another thing you would need to ensure is that your chain will be straight and not bent left or right (a.k.a chain line should be correct).

Are they a gimmick, work "meh", or extremely functional? Any experience with them?

I ride internal gear hubs (IGH) only, because they are extremely functional. I definitely prefer them over any deraileur system.

Why? Because IGHs have much better shifting than chain-shifts:

Ever stopped at the lights with your chain-shift in the wrong gear? Well, duh, you'll have to accelerate with the wrong gear in place. Not so with IGHs: You select the new gear, move your pedals back and forth once (only the pedals, not the bike!), it says "click", and you are ready to rock.

Ever found you needed to shift a gear lower on a steep ascend, and didn't have much speed to spare for the shifting? With an IGH, you select the lower gear, you release the pressure from the pedals for a split second (and I really mean "split", like a quarter second, or so), it says "click", and you are in the correct gear again. No need to reduce force until the chain has fully moved to the correct sprocket.

On a similar note, when you shift up during acceleration, IGHs virtually eliminate the shifting breaks in power output. It feels like driving a tip-tronic car over a manual shift car. Once you are used to the "click-click" shifting performance, you won't want to go back to deraileurs.

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The downsides of IGHs compared to chain-shifts are:

• They are a tad less efficient. That's why racers never use IGHs, they need the last percent of performance. However, that's definitely not an argument for commuting and touring.

• The spread of gears is a bit more limited. IGHs usually span a factor of approximately 3 between lowest and highest gears, chain-shifts for mountain bikes have a much higher range. I.e. on steep slopes you may find that you are missing a gear or two with an IGH. But it's still much better than having only one gear, isn't it?

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Update:

After getting some experience with a Rohloff Speedhub, I feel that I must mention that it deviates from my description above in some points:

• Shifting is not deferred, it takes exactly as long as you need to turn the controlling grip into the new position. This also works under some load, but the more pressure you have on your pedals, the harder it becomes to turn the grip.

• Efficiency is significantly higher than with cheaper IGHs I've ridden.

• Gear spread is significantly wider. Most other IGHs max out at roughly 300% spread, the Rohloff gives 500% spread.

There are other top category products that I have not tested, and which also deviate significantly. Most importantly the NuVinci which does not even have gears, it can produce any transmission factor over its entire range. I imagine that this can be a very sweet feature, and I did consider it for my investment. In the end, I decided that a high gear spread was more important to me than continuous shifting.

Of course, the Rohloff is the most expensive product in the market, so you must decide for yourself whether the above is worth the investment of a little more than thousand Euros. Cheap second hand IGHs may be obtained for 25 Euros, and they are already a very good choice. So, if you are on a tight budget, take something like the SRAM 7-Speed, and be happy with it. You should only consider the top products if the money is not really the issue. But if you decide to invest the extra money, I have found that it was indeed worth it. In my case, I mostly enjoy the higher efficiency and the small increment between the gears.

The easiest (if you haven't done it already) is to swap the complete rear wheel for the one with IGH like Sturmey-Archer or Shimano Nexus.

This is actually what I've done on my old city bike - I got a second-hand wheelset with Sturmey-Archer and roller brakes, cables etc. for about €50 and spent one afternoon fitting it.

If you would like to keep your rim than you need to source a hub and relace the wheel (probably a task for your LBS).

You ask about the experience - mine is positive. Once properly serviced those hubs are in fact maintenance free (though you need to adjust the shifter cable every now and then) and last forever.

I've serviced one SA hub that was about 30 years old (came from early '90s I reckon) - haven't spotted any wear inside. Mine (a bit newer) rides without maintenance for about 1000 km already (no idea how much it ran with the previous owners) and I can hear it needs cable readjustment.

RJ The Bike Guy has two videos on servicing those - there isn't any rocket science, only some basic bike mechanic skills and tools:
How To Overhaul/Clean/Lube Sturmey-Archer 3 Speed Hub - AW type
Overhauling 60 Year Old Sturmey Archer 3 Speed Hub