First off, lets talk about overall weight of the fly rod. Today, the trend in fly rod design is to make everything about the rod lighter. Getting a rod down in weight as low as possible seems to have become an obsession with fly rod manufacturers. They give many reasons for this, but unless taken to extremes, the weight of a rod is not nearly as important as it's action. Lighter is not always better.
A well designed rod that has some heft to it, without being so heavy as to feel like a buggy whip, has distinct advantages when fishing in normal situations. For instance, you can feel the rod loading the line much easier & can judge your back & forward cast with a lot less work & without looking over your shoulder. The heavier rod will also do more of the work for you when casting - especially when you first start your back cast as you pick the line up off the water. Furthermore, if the rod is well balanced, progressive in action, & has the proper amount of line guides in the correct locations, you won't notice the weight of the rod at all. What you will notice is the easy, smooth casting once you get your casting timed to the rod's action. If your casting a good designed rod correctly, your wrist & elbow will not be tired at the end of the day & you'll be casting as smoothly as you were that morning.
The second myth about bamboo rods that I want to address is the idea that they're slow in action. In fact, they are like fly rods made from all materials: some slow, some fast & everything in between. Modern manufacturing techniques, glues, & taper designs now allow the rod maker to produce bamboo rods with many different actions & characteristics. Perhaps you will notice that a bamboo rod flexes, or bends, a little more than a graphite rod when playing a fish. That is bamboo's strength. It has much more tensile strength than graphite (or fiberglass) rods - more than steel, even. Please don't equate the flexibility of a bamboo rod with a slow rod action. Many "parabolic" & other bamboo rod tapers flex considerably, but can be quite fast in action.
The truth is that there is no fly rod that will make you a great caster. Graphite fly rods can be awesome fishing tools & are very practical for fishing. A great fly caster will get the most out of any rod & will do amazing things with even a mediocre one. For the rest of us (myself included), we need to give ourselves every advantage we can.
Don't let what the rod is made of dissuade you from using it. If you haven't cast a bamboo rod, give it a try. If you are strictly a bamboo angler & don't understand what all the fuss is about graphite, pick up a good one & cast it. The bottom line is: a good rod is a good rod, period - no matter what material it's made of. What makes a rod a good one? You do. Get a rod that's designed for how you fish & cast & you'll have the best rod possible.