Within the Open Source world you're not going to get everything for free. For the functionality it provides, I am willing to plunk down $500/seat for my developers for Flexbuilder. This is the equivalent of 1 billable day for a contract Java developer in the US.
Quite honestly, HTML is still great for documents and websites. But for true applications, Flex is the way to go.
The biggest hurdle was the pricing model, something that Adobe screwed up on originally that hampered people even looking at the product. I remember from the Generator 1.0 days, extra zeroes just turn everybody off.
Something Adobe doesn't want to publicize too much is: YOU DON'T HAVE TO USE FLEX DATA SERVICES and this is where the expensive part comes in. This is the tough dilemna that Adobe is in with promotion. They still need to get folk on the Flex bandwagon and announce 'we've changed our licensing model' but still have to make money. They are fundamentally not a services company - they sell products.
If you are doing real enterprise dev the licensing costs should not be a factor. Otherwise if you already have a Java backend, likely using Hibernate and Spring, you can expose your data in a myriad of ways. We created generation tools that expose it as RESTful services and it is so easy to wrapper via Flex. I believe you can access your Spring beans directly as well though we have yet to dive into this fully.
I don't know about true usage numbers and hype (I wish they would do some more scientific analysis or surveys), but Flex has a larger community than OpenLaszlo in a shorter amount of time (please correct if wrong), it's RIA Flash competitor, which has some great stuff but is essentially losing the battle. I know there will be other sticking points down the road, but they will be surmountable.
Flex + Java is good company. The kool-aid tastes good and is free of poisons. Hop aboard, come down the rabbit hole, there is a new world awaiting you.
