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Hex-Rays Decompiler is an addition\extension for IDA Pro and I doubt it can work as a separate standalone product.
Please see "Prerequisites" part of Hex-Rays Decompiler Manual:
"The decompiler requires the latest version of IDA..."
https://www.hex-rays.com/products/decompiler/manual/prereq.shtml
But this question, IMO, should be addressed to the Hex-Rays Sales Department.
I suggest you look for decompiled code samples posted on the internet. That should give you a good indication of the decompiler's capabilities.
You can interactively decompile. Or also decompile in bulk. It decompiles a 22M binary containing 57k functions, totalling about 4.3M lines of code in about 1 hour. The decompilation can also be modified using pytbon scripts.
With some tweaking of variable and function types you can get very readable code. Also of c++ or objective-c with lots of virtual function calls.
Short answer: No, there is no other interactive decompiler, at least not for native codes.
Long answer: The Hex-Rays decompiler was created with the idea of interactiveness while all the other decompilers for native code that I know (I'm not talking about Java or .NET) were created as batch tools. The closest to a half interactive decompiler is Snowman, but is not really interactive (unless selecting which function you want to decompile when using the IDA Plugin is considered interactiveness.)
There is a toy "decompiler" (if the name is appropriate, as it's more a translator to pseudo-C than anything else) in Hopper but is neither interactive.
Boomerang, REC, and all the other abandoned decompiler projects that I know, were created either as command line batch tools or had a GUI just to let you select a binary.
So, as said at the beginning: the only interactive decompiler for native codes is the Hex-Rays one.
PS: I don't know why people is not focusing in the question instead of proposing their favourite tools... A disassembler is not an interactive decompiler.
The plasma project seems to have an interactive decompiler: https://github.com/joelpx/plasma
From the documentation:
$ plasma -i tests/server.bin
>> v main # or v 0xFFFFFF (replace 0xFFFFFF with the address of the function)
# You are now in visual mode. Press P to define the function. Then press TAB to decompile the function into pseudocode.
# You can hover over a variable using your arrows keys and press R to rename a variable. You can press X to see all xrefs to the variable.