I hear a lot of talk about using ultrasonic cleaners to clean engine parts.... How do they work? Are they worth it to buy?

Yes don't spend no more than 100.00. and make sure all rubber is removed from the carburetor

Vibrates, pulses and heats up to remove dirt or grime. When I first bought mine I wish I would have bought a bigger unit. So got a 6 liter unit off eBay for less than $100 last week

Does anyone have a harbor freight one?

Before Sonic cleaning:

 
After:

 
 
If you put a piece of aluminum foil in the machine and run it enough, it will literally put small holes you can see thru it.  I generally just use dish soap with water does the trick.
 
For aluminum carbs, the first few sonic treatments will clean up the aluminum considerably.
Much more after that, it'll DULL the finish. 
 
 
You decide....

That gives me what I want to know Scott, thanks I am deffenatly going to buy one. I don't really care about the finish because I use a sandblaster right now and it eats down to raw metal. Have you ever cleaned  fouled plug? How do they do?

I've never put a fouled spark plug in the machine as I clean spark plugs
in a specific spark plug cleaning tool (see below).  That uses media and they come out spotless.
 
It's not this exact one but very, very similar but from Harbor Freight (I didn't see it on their site just now):
https://www.amazon.com/VCT-Pneumatic-Cleaning-Blasting-Abrasive/dp/B084SVZ4W8 
 
Every once in awhile, I'll come across a plug that'll fire OUTSIDE the cylinder and
once in the head will not fire....  That plug gets pitched immediately.  I do keep
known good ones for testing..
 
 

I have a cabinet sandblaster and it works so I will probably continue using it, I want a ultrasonic mostly to clean jets and small passages that a sandblaster would destroy.