it has python in it plus a lot of cool plugins not to mention immunity's active support , but originally it came from Olly's source code . olly and its scripts are famous in cracking community but immdbg designed to be a dynamic vuln research utility
Immdbg has also fixed some of the bugs found in olly which some packers try to exploit. On the down side, it has a slightly annoying ad rectangle up at the top.
However, the statement of 'being light years ahead' could only come from a python fanboy. ;)
I'm not sure if all the Olly Plugins will work fine in Immunity. Also I believe there is a Python plugin for Olly anyway, so a lot of people (hardcore olly fans) haven't switched as they don't see any benefit.
I prefer Immunity Debug over Olly. But I'm a late comer, so I was not already overly familiar with Olly.
AFAIK, the only new features ImmDbg offers over Olly is Python scripting and function graphing, both of which are already supported in Olly through plugins. There are also plugins to fix the numerous bugs Olly has as well. And like b0ne said, there are ads in ImmDbg, which is really annoying. The fact that the hundreds of plugins already made for Olly don't work with ImmDbg pretty much ruins it for me.
To be honest, if Immunity really wanted to benefit the community they would've just released the Python and graphing features as plugins for Olly instead of buying the source code from Oleh and trying to pass it off as their own "new" debugger. Besides, ImmDbg will pretty much be irrelevant once OllyDbg 2.0 arrives.
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