I find decent stuff quite often, usually after completing story missions and allowing them to "level up". I couldn't imagine grinding for a good shield or class mod instead of checking the vending machines.
They are only really good for when you just reach the area or if the items are up to your level. Otherwise if you are doing the sidequests and whatnot you'll almost always be looking at items 3-4 levels below you and that always sucks.
You have to know how to shop - most shops will sell under leveled stuff if you're doing all the side quests.
All of the ones I've seen sell in a 2 level range (e.g. 7-9). If you're level 13, that shop isn't going to help you. If you're level 7, it'll give up-to-level gear, and that's often more important than oranges.
It's rare for me to even find greens in the gun machine, and it's usually just the item of the day if any at all. I have never seen a blue, nevermind a purple or orange. I use to find oranges in vending machines relatively often in the first game. Got my best Hellfire from a vending machine before DLC came out. I see green shields/class mods slightly more often, but still have yet to see a blue or purple in the medical machine.