It was the easiest to implement and control, had the most features and I had working map with most of the effects I wanted in an afternoon. However, I had to stop using the Yahoo! API because of 3 major problems.
1) The Yahoo map breaks if scaled to anything smaller than 260px high (if someone knows how to make is smaller I stand corrected) 2) You are limited to 50,000 hits per month for your API key. 3) It is strictly not for business use.
Due to the lack of features or stability or just the fact they looked naf I didn't attempt some of the other 3rd party APIs I found and instead tried Google.
Google can scale down as small as you want, does not have an access limit only a URL restriction which means you need a different API key for each domain you want to use the map on. Google also states it is for non-commercial use but as long as your site is not charging for using the map or any of it's features then this is not a problem.
Google's API is no doubt powerful and has a lot features under bonnet. Just getting at them seems to be harder than need be. In the end we manged to put together quite a handy airport finder.
You can view the finished homepage here: http://www.atlarge.com/
Comments and suggestions on further tweaking are welcome. Much credit to Alan Milford for doing the Ajax magic for me. The airport markers appear once you zoom in close enough to an area. Want to know if there is an alternative airport near the destination you are trying to visit - now you can.
In addition to web use the concepts and syntax used is almost identical to the style of action script in flash and the scripting used in Neverwinter nights world building tools. I've purchased O'Reilly's definitive guide - will I become a scripting wizard? watch this space...