As an owner of a 12" Grizzly J/P with a carbide helical cutter, I would certainly recommend it for the serious hobbyest. It replaced an 8" HSS knife jointer and a Dewalt 12" lunchbox planer. For questions in your original post,
1) I feel the carbide head is a large improvement over the HSS knives. 800 - 900 board ft of hardwood through it without a noticeable dulling of the head. Having the J/P combo allows for the head to be used for both jointing and plaining. In Dec I ran some very figured 8" - 11" curly maple boards through the jointer and planer functions with very little to no pull out.

2) You are correct that the J/P limits the length of of the jointer bed and this is indeed something to be considered before purchase. Edge jointing is never an issue but face jointing 10' and longer boards can be tiring. Thankfully the longer boards is a small part of what I work with.

3) Over the last 1 1/2 years the grizzly machine has performed well and done everything I have asked of it. Short of the beds warping or motor failure, I do know of anything that could go wrong with it that I could not fix. Before purchasing, I looked at all of the models on the market and picked the one I felt offered the most value (bang for the buck).

4) I have used the 12" jointer width more then I thought I ever would. Once you have the capability, it is hard to go back to the 8" or 6".

Just my 2 cents...

Mark