Okay, time for my two cents as a FRONT end dev.
I absolutely love the Joomla community and the extensibility of Joomla, and it is a bit easier out of the box for a user. BUT--
The core code of Joomla dictates to me, the front end dev, what HTML I have to use unless I want to hack core to make it work differently (not good at upgrade time, oh no! Not to mention that we’re talking about a great NUMBER of core files that would need to be hacked.)
It is IMPOSSIBLE to build an accessible website in Joomla. It is also IMPOSSIBLE to build a tableless website in Joomla.
MANY of the plugs/mods/mambots which, from one point of view are Joomla’s strength, generate TAG SOUP on the front end, causing huge headaches and angina to the front end dev- me. I find this to be a huge weakness since I have better things to do than hack around trying to get broken code to look good in all browsers (especially IE.)
The editors which are easily integrated into Joomla are also guilty of sometimes (often) generating non valid front end code, also adding to the above mentioned headaches.
Now, ModX--
On the downside- it’s a young application that’s not yet very extensible, though I’ve seen great improvement on this front since it’s been released.
It can be a steep learning curve for a strictly front end dev (I am so php challenged it is NOT funny.) However, if set up right, this can be a VERY easy application for the user despite what others have said. I’ve succeeded in doing that for a client, though my hair went half gray in the process . . .
On the UP side-
ModX NEVER dictates to me what html it will generate. I have complete control over the frontend. I can make my layouts SING in this web ap! And with minimal hacking to get things working cross browser since I am the one dictating the frontend code instead of the web ap doing so. Not so in Joomla.
The community here, though smaller, is EVERY BIT AS KICKASS as the Joomla community. Even better is the commitment here that I see to write an application that is totally capable of w3 and wai compliance. Actually, it’s already been written. Compliant websites are already possible with ModX (and have been from the beginning, I think) whereas mambo, and then Joomla have been talking about the "coming release" of their accessible version for well over a year now, with no release date in sight.
That commitment to coding excellence and forethought to the future extends into the extensions/plugins/mods/snippets/chunks made available through the community to extend ModX. Though I do admit that my use of ModX has been limited and there may be more, I personally have only found ONE snippet that generated non validating code (minor and I was able to fix despite being all thumbs with php) and I think I may have grabbed that one off the etomite site
The forum integration and other integrations, mods and extensions can’t come soon enough for me. I really don’t want to have to template to Joomla or the others since it’s so much eaiser for me to work with ModX. The couple clients I’ve had who are using it agree. It took them a bit of learning to use the admin interface to create documents, but I had them up and running editing their initially created docs within about five minutes with quickedit. They were tickled and so was I. Even better, with some wanking on the editor, it’s generating (mostly) compliant code and what noncompliant code is created has been put there by the client, so there have not been any whining from them about broken pages or stuff flying every which way in the layout. That makes me a VERY happy camper!
I’m still wondering why people say ModX is hard to use -- I STILL, AFTER HOURS AND HOURS CAN’T USE CIVICSPACE/DRUPAL. I can skin it- I just can’t use the SOB. And though ModX is a bit challenging for ME to set up, my clients, who, though reasonably intelligent are ’puter challenged, are loving this ap.
Hope that helps.