It's free to use for people who get less than 2,500 hits per day - wouldn't we all want a hit average like that! :)
I believe these are better than "Guestmaps" because it will automatically add the location of the visitor to your map by placing a dot. Guestmaps usually require the visitor to physically add their location.
You're students will really feel like they are "world famous on the web" when they see lots of new dots appearing on their map. My class also enjoys finding the name of the countries where the new dots have appeared.
Click here to download a step-by-step PDF document that I have made detailing how to add a clustr map to your Blogger Blog.
Further information about inserting a ClustrMap into Blogger is available here or email me if you get stuck.