The most valuable feature in Celoxis for our PMO is its API. We've been able to do intensive integration work on less than 4 months. This has helped us to use a very user friendly adoption path, since people are not seeing disruptive changes in the way they work. What they see is a very gradual transition between their traditional tools and processes to the new ones on Celoxis. For example, for reporting time logs, we used the API to integrate Celoxis with the same GUI our consultants have used for a long time. They saw little differences in that we integrated the time codes catalog into the GUI, but as a PMO we've been able to use the time information in a very productive way in Celoxis. Now, we are gradually moving some users to entre their timesheets using the powerful Celoxis GUI.
Another very powerful feature is the workflow engine. We are also using that one a lot. Be it for managing risks, change requests, issues and project requests (the built-in original apps) or our custom made apps for agreements, business goals, stakeholder management and deliverables acceptance. And we can use all of them using Celoxis GUI, e-mail or the API.
In terms of common project management features, like the Gantt, and the way tasks are added, managed, updated, related, scheduled, etc. Celoxis performs as expected.
The web GUI has some limitations in terms of there not being a way to zoom, so you get an standard sized view. Personally, I like smaller text and a better use of the visible space, but there's no way to do that on Celoxis.
Another thing we don't like is that a lot of the web views are shared among all users. For instance, I set my dashboards the way I like them, but at the same time I am modifying the for all users. Also, you can create different dashboards but they share some of the widgets configuration. I currently have 3 dashboards. One for those projects I am accountable for their performance and success (my projects), a second one for projects in which I act as a PMO mentor for the project managers, and a third one for all the other projects. Since my involvement is different, I need different information on the Project List widget in the dashboards. But, if you change the column list on the widget on one dashboard it is modified for all of them. I don't like that at all.