<p>I'm working on a project that currently has zero users but we would like to scale up to potentially hundreds.  Currently we are running on a MySQL database with AMFPHP interacting with Flex.  We used Flex because of its robust graphic features (important to this project) and because the initial developer (not me) already knew ActionScript.  We are currently using AIR but might switch to web-based Flash at some point.</p>

<p>My questions are:</p>

<ol>
<li>Is Flex a good tool for a project like this?</li>
<li>What are the main limitations of Flex that we might encounter?</li>
<li>What are other development platforms we might want to consider?</li>
</ol>

<p>Thanks.
- Dave</p>

Flex has no inherent scalability problems, however if you have a graphic intensive application, proper serving of these resources might be a problem, but that has little to do with Flex.
The only note-worthy and likely platform you won't be able to run on is the iPhone (no flash) and some older non-flash mobile devices (although most support Flash-lite nowadays)
As for alternatives, if you are Graphics heavy, and don't mind the iPhone, then Flex is good if not best cross platform solution besides using pure HTML technologies, the trick here is HTML alone can do 99% of what Flex can do, but if your App requires the missing 1% then you're out of Luck, also Flex will reduce crossplatform and most browser compatibility issues. So it might make your work more productive.