I just stumbled upon a really, really great post on Steven Sinofskys Blog.
His article is about the challenges of cross-platform development in general, and he brings up some rather good points on why some approaches will eventually fail.
I'm pretty sure he doesn't even know about FireMonkey, but this is what he has to say on cross platform libraries in general:
One of the most common approaches developers attempt (and often provided by third parties as well) is to develop or use a library that abstracts away platform differences or claims to map a unique “meta API” to multiple platforms. These cross—platform libraries are conceptually attractive but practically unworkable over time. Again, early on this can work. Over time the platform divergence is real.
And then he continues:
Worse, as an app developer you end up relying on essentially a “shadow” OS provided by a team that has a fraction of the resources for updates, tooling, documentation, ..
It is important to keep in mind that the platforms are evolving rapidly and the customer desire for well-integrated apps (not just apps that run).
The very last point is what I already stated in my own post on why FireMonkey is wrong. I didn't even write about the even more important first ones. And this only are quotes from one paragraph where he thinks about cross-platform libraries.
I strongly suggest that you take a few minutes and read what Sinofsky wrote about cross-platform development. And then, if you currently feel that FireMonkey could be the right tool for you, try to understand his points and re-think your position on cross-platform tooling. I'm sure you will see that FireMonkey can't be the right tool for you - or anybody.