This spring at Flex360 in Milan, I presented a session about working with data in Flex Builder 3, and I was asked a lot about LCDS versus BlazeDS. When I asked questions about the use case, I found that most of the time the only reason to use BlazeDS instead of LCDS, was that BlazeDS is Open Source and thus free. Months later, I had a deja vu experience in Holland. It seems that very few people know that there used to be LiveCycle Data Services ES Express (for LCDS 2.5) and there is a LiveCycle Data Services ES Single-CPU License (for LCDS 2.51). This license, basically lets you use the full version of LCDS in production with this limitation: you have to deploy the LCDS on a machine with a single CPU (a multi-core CPU is counted as a single CPU) and a single application.
So, if you need the LCDS features and you can work within the license terms, then go and enjoy LCDS for free. I can think of a lot of applications built for small and medium intranets (CMS, mini ERPs and so on) that probably can be run successfully on a machine with a single CPU. And if the application grows over time, probably your client grew as well, and now they can afford to buy a license for multiple CPUs (they had the chance to test the technology and see its value gives and what problems it solves).
You can find more about the license here (click on “What happened to LiveCycle Data Services ES Express” for example) and this is a great post that explains the LCDS and BlazeDS products and licenses.