I started with Mambo back a couple of years ago when I first got my DH account. Loved it. But was really disatisfied with any help I needed. Most of the time I posted questions on the board it took days (if I was lucky) to get a response. And if I was lucky the response was either not useful or insulting.
When DH provided Joomla I did my research and posted a couple of quieries on their board. Amazing difference. Not only did I get a quick response, but helpful and not condensending.
So, I switched my origional site from Mambo to Joomla. Backed up the data deleted the site, and Click ... Zoooom! Joomla was installed.
So far every mod or component you can see for Mambo you can get for Joomla. And a lot of companies have switched to Joomla from Mambo.
I had a need this week to do an even mod with paypal for a client. Found about 3 vendors who want 20 - 100.00 for their product. A quick search on Joomla.org extension and I found a component for free.
Joomla now has about 450+ extensions and it is growning. Not to mention that documentation is slowly comming on line.
I never really did find good docs for Mambo.
Another plus is currently most Mambo extensions will work. The word is this will continue until the next major Joomla and/or mambo release.
I hope this helps.
For the money (what money) you can't beat Joomla.