I've used one for the last 2-3 years. Indispensable if you have out-apiaries and the logistics of clearer boards don't suit.
Pro - single visit clearing and collection of supers possible.
Con - there always seem to be a few stragglers that can be a pain in the extraction room, not viable on a wet cold day, bees can be surprisingly good at holding on, especially if any unsealed patches.
I've not noticed any damage to bees by using a leaf blower. In an apiary of 10-12 hives it looks very chaotic for a few minutes afterwards but within 20-30 minutes there is no outward sign of anything amiss.
I use a standard leaf blower, I convinced myself that it would be useful for the garden as well. Importantly for the honey, the exhaust is separate from the blown air. The standard attachment generates a strong enough blast of air to be effective without modification. There is an argument for buying cheap blowers as they will inevitably get blocked with bees/honey/propolis but I have yet to have that problem.
I still use clearer boards where it's possible to visit an apiary twice in one day and then use the blower to clear the laggards.