Saw them in Reykjavik a couple of years ago. We went in late February, which wasn't a bad time to do it: it wasn't too cold and dark but the nights are still long enough that you have half a chance of seeing them.
It's completely a matter of luck, though. We went out on a Northern Lights boat-trip out into the harbour away from the city lights, and barely saw anything - just a grey splodge in the sky. Then, a couple of nights later, we were walking between a bar and a restaurant, happened to glance up, and a major light-show was just kicking off. It lasted 40 minutes and it was fantastic. Tourists were high-fiving each other in the streets, while the locals just strolled by with "dhera, this happens every week" expressions.
I'd advise going somewhere where there's plenty to do anyway, and you might have a chance of seeing them, rather than going on a specific Northern Lights trip where you might wind up disappointed if they didn't show. We were having a great holiday anyway, so seeing the Northern Lights was more an added bonus.