I am looking for some insight into real world experience of upgrading a Tridion 5.2 SP1 system to 2011 SP1. What must be changed or will definitely break (GUI extensions for example), what areas are risky (for example Broker query code), what could benefit from a rewrite (eg event system).
Here are some notes that I wrote in the past when upgrading to Tridion 2011 GA, but the issues will probably still be similar. This covers some more general issues as well as some technical ones.
Do you use any Custom URLs (Opened as popups from the Tridion CME GUI when clicking on the field name of a Component)? If so, then I found that a couple of these had been removed/superceded when upgrading to 2011.
You will probably have to upgrade any dynamic content delivery code. Although the documentation says that it is backwards compatible, I've had some queries that would not run using the old code and Tridion Support recommended that I upgrade them to use the new 'Criteria'-based code.
Tridion 2011 will make taxonomy hierarchies and content categorisation (rather than using Component metadata) available in Categories & Keywords. Although you may not need to use this initially, it may affect future decisions.
If you do not currently, it is important to NOT 'includeKeywordBranches' when performing Taxonomy Criteria searches as this SEVERELY affects performance! (This may now be fixed in SP1)
I have heard of problems upgrading WebForms in the past. I have not been involved in this, but it sounds like every webform created had to have some sort of manual edit.
Do not under estimate the amount of time needed to re-publish content if you do start again with empty Broker databases.
You may wish to consider moving any 'per publication' configuration that was in the old broker config file in to the Publication settings (in the CME) if possible. Otherwise you can end up with very a very large storage config file in 2011.
You will need to consider the possibility of a content freeze, double entry or a later content delta upgrade between the old and new systems.
The old Powertools are not available in 2011 so if there is something that you need from these then you may have to re-write it yourself. There is a community effort to recreate these for 2011 but these do not yet cover all of the functionality offered by the R5.2 ones (although they do include some cool additional things).
Do you have any GUI customisation in the old system? If so, this may need to be rewritten too.
As mentioned this was for GA and not SP1 so please feel free to add comments or edit where this information may now be inaccurate or unclear.
I know you asked for "real world" feedback, but I still feel it's important for anyone looking for this information to consult the wonderful "What's new in SDL Tridion" section in the documentation (login required).
It doesn't just list what is new in the current version -- it goes all the way back to SDL Tridion 2009. So it has all of the information you would need when upgrading from 5.3 onwards.