Thunder is a really useful tool, and is very good for creating basic designers for standard control implementations.
I agree - if I were new to Sitefinity and/or designers, there would be absolutely no contest in where to start creating a designer for a widget - Thunder.
It basically appears to work by reading the public/browsable properties of the control (i.e. those listed in the property editor) and then generating a boiler-plate designer with lots of // Your code goes here // type of lines - and it does a good job of that.
However, things get less certain the more advanced/specialized your environment and/or the more experience/knowledge you have with the base technology... and like all code-generating tools, Thunder has its limitations.
The biggest issue with 'doing your own thing' and manually creating designers is the assumption that you are fully conversant in asp.ajax as the primary client platform.
Prior to Thunder, this assumption made developing even the simplest of designers, a serious challenge for people without that background, as the learning curve was near vertical, and Telerik (quite rightly) didn't attempt to teach developers that base technology.
This made going from V3 to V4/5 quite difficult for the average dev with little prior need of in-depth asp.ajax experience, as half of the required information is missing - and getting to grips with it was made all the harder by Microsoft's decision on the future of asp.ajax.
Personally, starting with V4 came as a shock to find how much I needed to learn, and it was nearly 2 years of pain - partly because I'm so stupid, but partly because the documentation was so sketchy on both Telerik's and Microsoft's sides.
I have custom environment at both server and client levels that Thunder really struggles to deal with, let alone enhance, but that's not unreasonable and I'm accept my trade-off as a choice of development priorities.
However, having got to the point where I no longer feel I'm just a passenger in the development cycle, I'm not about to give it up... LOL.