THey've worked really, really well for my family. DH is a cyclist and brushed it off years ago when I first mentioned them. THen Mark Paul Gosselaar mentioned using them with his kids in a cycling mag (DH is a cyclist) and suddenly he was interested
The last two homes we've owned have been in very small subdivisions with a cul de sac and flat streets. I think it would be tougher to learn if you live in a hilly neighborhood or don't have a good area for riding regularly (since kids have to build up some speed over a bit of a distance in order to really glide). We had a rental in between those homes for a few months and it was also flat (no cul de sac) but not heavily trafficked. That's where DD transitioned to a pedal bike, but she was riding the BB while we were there as well.
My DS1 was always a more cautious kid; he got the BB for his 3rd bday. We bought a Puky from the gliderrider website, but that site had some customer service issues and so forth. There are a LOT more options now than there were back then. That site had videos up of kids riding, and you could see how they progress from walking to running on it, and then gliding on it. I think that helped him get it. WIth my subsequent kids, it was easy to teach them as the older kids could demonstrate it. My DD and DS2 were 2-2.5 when they were using the balance bike pretty well. They are less cautious and also had older sibling(s) to model how to use it.
My DS1 did have one pretty bad crash on it. Even a very small incline can have them building up a lot of speed. He was coasting down a very slight incline but going very fast. He tried to slow using his feet, and that made him sort of wobble/swerve and he crashed and had some good scrapes on him. It was stll worth it, but I'd just make sure you are aware that when they build up speed, they can be moving shockingly fast. It is easy for them to get a good bit ahead of you on a walk, etc. even as a 2-3 yo (yikes), so just be aware of that (and kids that age are not going to be attentive to cars backing out, etc.).
We also had two neighbors learn to ride using the BB. In both cases the kids had been on 2 wheelers with training wheels for months to years but hadn't transitioned beyond that. A few days of messing around on the balance bike taught two of the neighbor kids around their 5th bdays.
My kids all still used a trike to work on pedaling. At some point they were able to mesh the balance bike skills and the trike pedaling skills to move to a 2 wheeler with pedals.
Mama to DS-2004
DD-2006
and a new addition-ds born march 2010