Here's a tip that I wanted to share within everyone here. It may be
known but I only found out about it today and wanted to share.
If you ever get the dreaded "Invalid Reply from Server" message from
AjaxCFC, spark up Firefox/FireBug and call your site from your local IP.
This will give you the actual message that DWR is sending back.

Put a stop point at about line 517 (or immediately after
"batch.req.send(query);") of engine.js and look for the "batch" object.
In the there you'll find the "req" (XMLHttpRequest request) object which
has a standard attribute called responseText. That will give you all of
the details of your error but for security reasons, it only shows it via
your local IP address (127.0.0.1).

Rey...

I was trying to speed up a process of mine that involves a large amount of string concatenation.

In other words, <cfset str = str & someOtherString>

Turns out there's a much faster way.  In my case, I was dynamically generating an data file from a query.  Each row had let's say 50 fields.  Generation of each row required 3 string concatenations at a time (maybe more, depending on how it worked when compiled to the servlet).  Do a drop of 5000 records, and well, that's a lot of string concats!  750,000 at a minimum.

Someone on cf-talk suggested using a StringBuffer might be wise because Coldfusion probably creates a new string object every time you do <cfset foo = foo & foo2>.  So I decided to write some code to test it out.

Sure enough, using the java StringBuffer class is quite a lot faster.  On my local machine, I ran the following code (see below).  The CFML method completed in 89 seconds, while the java StringBuffer method completed in only 28 seconds - about 1/3 of the time!

Here's the code, if you're interested: