SP23 was released two weeks ago, it's kinda early for deployment to production systems. At the test systems it behaves nicely and till now have find no issues but we have not began extended testing yet.
About the second question, the SPs are cumulative, so really you need only to install the latest SP.
My advice is to migrate your testing environment directly to SP23 and start testing.
I have several customers already running SP23 in there testing environment and no issues yet.
And as George mentioned do not directly migrate your production environment to this SP, first do some intensive testing your self and check back here on the forum to see if anybody else has any problems.
I since found that a Java upgrade is required for SP19 and above. I'll continue with the wade through the doco and posts.
Most of the info on SP23 looks like bug fixes so hopefully if not too much functionality introduced maybe fewer bugs introduced?
ITSM008978 is interesting. There was a work around for using UI notification messages on value changed where you start the rule as a "Before Item is changed", set the User Notification then go back and change to "When Value has changed". I believe SP18 or 19 stopped this from working? Do I read ITSM008978 correctly - it is now OK if the message is "informational" (versus "Error/Warning"). Currently we have a few UI rules done using the old workaround and would not like to lose them.
Not sure if you have any rules which sent emails when a fields it updated. eg. Customer Update.
e.g.Rule is When Customer Update is not empty Send Email Clear Customer Update.
After SP21 the sending of email becomes asynchronous and there is no guarantee it is sent before the next line in the rule clears the field.
I have had to modifiy this type of thing to use an extra field and an extra rule. The first rule copies the Customer Update to the new field and then clears Customer Update. The Email rule fires off the new field not being empty and sends the email, but unlike the original rule does not clear the field.
Also note that ServicePages go to Tomcat 5 somewhere between SP17 and SP23. This is described in one of the PDFs.
David, I think you could still get caught with this as the email sending is asynchronous and does not require any rules to wait for it. If it gets delayed for any reason (e.g slow resonse from SMTP server) the rules will fire independantly and could complete before the email gets sent.