I use intermediates for low water and smaller streamers (what I call small) but that's about it. Not bad for hitting bank structure but there's no way to get a fly on the bottom with them during generation. Most of the truly BIG fish I have pulled up have come from bottom dredging. This kind of fishing will cure insomnia. But, if you want to have a chance at the biggest fish in the river then sink-tips, full sinking (hard to manage), or heavily weighted flies are the way to go. Casting is a problem with a long leader and a heavy fly/flies so you don't want to be casting floating line and heavy flies frequently. If they are dead drifted then it's not as much work.
Sink-tips work best for what I do. If they're giving you trouble casting them then you may look into using a softer rod. When using sinkers you want to feel that rod bend and seem like you're launching the line. Parabolic rods work best for less aggressive casting. The discontinued Sage DS2 is an awesome rod for sinkers (look on ebay or try to find an action like it.) Also, be careful that you're not using too much rod for the line you're using. A 150 grain is very ill suited for a fast 5wt and heavier rod. I use 150 grain lines on 4wt's. 5 and 6 - 250/300. 7 thru 9 - 300 and up. Work on your timing if a softer rod doesn't work.
If the fish are actively looking up for prey then an intermidiate is fine. Intermediates are great for low water fishing when you don't want to weigh your flies. They're butter easy to cast but they don't get deep enough.
Thanks for the invite, Phil. Keep in touch with me. Certain times of the year are better than others.