Hi, I'm an experienced commuter who's ridden hybrids and road bikes for some time. I'm thinking of getting either a commuter with a geared hub (Nexus or similar) or a 29er. I've heard that it's hard to change a flat on a bike with a Nexus hub. Is that so?

https://www.youtube.com/watch?v=rGEXjpXtw4g here is how to change the hub you are talking about. You should go with Sturmey Archer though

I just bought a Swobo Baxter with a Shimano Alfine 8 speed hub. Some riders say that once you have the right tools and practice disconnecting and reconnecting the shifter cable a few times it's not that difficult. At least that should not be the sole reason not to buy a bike with a Shimano IGH.

Now in my experience - and common knowledge among IGH aficionados - Sram geared hubs have a far more user friendly shifter/hub connection. It takes just a minute to get the wheel ready to remove. But with that said; the Shimano Alfine hub is so much smoother and quieter running than my Sram 7 speed I am glad I went with it. Although I can't speak for the newer Sram i9 hub, not having ridden it.

One thing to consider is that if you are just patching your tube, as opposed to replacing it, you can do so without removing the wheel from the bike. If you need more clearance, you can drop the wheel without removing the shift cable. But best learn the right way. I'll be checking out the video link posted by sooprvylyn.

All the best. DanO