A few personal and non-scientific comments, based on also having to learn Siemens PLCs after experience with A-B:
1) DO NOT approach it asking questions like "What is the Siemens equivalent of XIC?". Instead ask your questions in the format "How do I create a NC contact with Siemens?". This is a subtle difference, but it will save you endless aggravation. Siemens has a way to do every task that an A-B can do, but there are often no exact equivalent instructions.
2) Siemens was developed with the European attitude that programming was to be done by engineers, while A-B was developed with the American attitude that programming could be done by technicians and electricians. Keeping this in mind will help you understand why Siemens does some things the way they do, although it won't help much with the how.
3) Siemens programs in ladder logic, but they don't really mean it. (A-B has some alternate programming languages like SFC, but they don't really mean that either.) Therefore, when programming Siemens I suggest that you do your base structure and sequencing in ladder but make frequent calls to functions.
4) The HMI and PLC software are integrated, more or less. One thing that annoyed me was the time lag between starting the HMI software and it being done loading. I learned to time the start of HMI modifications to coincide with my need for starting a nature and coffee break.
5) The worst part of Siemens, in my opinion, was the clunky and lethargic online monitoring - especially viewing blocks of data. I never did get it all figured out, but got to the point where I could look at data well enough to debug and tune, although not efficiently.
Echoing some of the above responses, I didn't have any formal training in Siemens, so some of my problems were no doubt my own fault. On the other hand I didn't have any formal training in A-B either, and I picked that up with only minor levels of cursing.