My own background is as a .NET developer using mostly C# and lately ASP.NET MVC. I've also been a Dynamics NAV developer/consultant/architect for about 3 years now.
The Dynamics NAV world is quite a small one and to be honest it's neither growing nor shrinking. I've heard of a few places recently moving from other ERPs to NAV and just as many moving away from NAV.
I attended a briefing at the Microsoft Executive Briefing Centre in Vedbæk (Denmark) earlier this year and met with the Dynamics NAV GM as well as some Dynamics NAV PMs and developers (i.e. the devs that write the actual NAV app) and the roadmap they have for the product is really exciting - there's going to be a huge focus on HCM and improving some of the financials over the next couple of versions.
In terms of day to day working with NAV it's a bit of a paradigm shift alright. As you mention, the IDE is absolutely terrible. They only added syntax highlighting recently and there's no real intellisense or any of the modern conveniences IDEs today offer. Having said that, you can do some tremendously powerful stuff by combining native NAV objects with add-ins, etc. and they've really improved some of the scaffolding tools to help with development.
Financially, NAV developers do pretty well because they are reasonably rare. NAV solutions architects and consultants do even better. You profile doesn't state where you are but I know in Dublin the starting salary for a NAV developer is around US$60k and in London it's about US$65k.
The job market is much smaller than that for C#/.NET devs but jobs tend to be a bit more secure and there is a growing market for customers hiring in-house NAV devs rather than only partners/providers hiring devs and consulting them out to customers.
I personally wouldn't see it as a binary choice between C# and NAV. Sure, your title may be NAV developer but if you're using some of the later versions of NAV then you may still do a lot of C# development writing add-ins, etc. It's also a fantastic opportunity to brush up on your SQL knowledge as writing/optimising well performing code in NAV requires a reasonably deep knowledge of SQL and how queries get executed handled right the way through the process.
Do you have any more specific questions?