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There are two types of flybar-less- electronic and mechanical. Mechanical is mostly for scale, and I'm guessing the OP is more interested in aerobatics / 3D, so lets ignore mechanical for now....
Advantages of FBL:
1) Less drag, therefore more power to the rotors.
2) More responsive, and more easily "tuned" to the pilots preferences.
Disadvantages:
1) MUCH more expensive! (FBL unit, heavy duty servos, etc)
2) Takes longer to set up initially - some units require access to a computer to do it properly.
3) Prone to tipping on take-off.
4) Servo damage more likely in the event of (3), or indeed any blade contact with a solid object.
I've recently built my first electronically stabilised FBL heli. I can normally set a flybar up from scratch in about half-an-hour. The FBL unit took me nearly all day, with constant references to the user manual to set it up properly on the computer. I accept that this was partly due to my unfamiliarity with it, and partly that it is known to be a complex unit to set up! Not all units are that tricky. From a flying perspective, you need to be MUCH more careful with take-offs and landings. I manageed to strip all my servo gears (metal ones!) due to a slightly cack-handed take off, despite there being no other damage to the model or blades! Once in the air, there is less "sneeze factor" than with a flybar - but the same is true (more so, in fact) of mechanical FBL, which I've been flying for years.
So its horses for courses. If you are into full on 3D or FAI flying, FBL is the way to go. If you are a sport flyer, its a lot of money and hassle for little gain.
Multi-blade heads:
Again, there are two types: Those intended for scale, and those for aerobatics. Scale ones are often fully articulated (as per full-size), and as such, don't always need an FBL unit. Aerobatic ones tend to be rigid, and unless very big and turbine powered, will almost certainly need an fbl unit to make them flyable. I'm assuming the OP is talking about aerobatic use again.
Multi-blade heads tend to have more inertia than two bladers, which makes autos better. They also seem to run smoother. - a bit like the difference between four, six and eight cylinder car engines! Odd numbers of blades seem to work better than evens, many full size machines having an odd number of blades. Jim Morley used to say that he could never get his 4-blader to fly as well as his 3-blader, despite them being otherwise identical! Tha manufacturers would have you believe that a multi-blade head will respond quicker, but the maths doesn't seem to support that. The "following rate" seems to be dictated mainly by diameter, weight and rpm, rather than the number of blades. The improved response time - if it exists - must be down to other factors. However, the added smoothness alone is a big bonus.
They are, however, a pig to get tracking accurately! Its not simply a case of tweaking a blade and seeing if you make it better or worse, as with a 2-blade. You have to identify WHICH blade is out as well, and that is often a 2-man job! If you are lucky, and very careful with your initial bench set-up, you will probably get it pretty close immediately. However, with a bit of wear and tear, expect the tracking to start to go out, and that's when the nightmare begins!
Just my 2p worth! Other will probably disagree!
Advantages of FBL:
1) Less drag, therefore more power to the rotors.
2) More responsive, and more easily "tuned" to the pilots preferences.
Disadvantages:
1) MUCH more expensive! (FBL unit, heavy duty servos, etc)
2) Takes longer to set up initially - some units require access to a computer to do it properly.
3) Prone to tipping on take-off.
4) Servo damage more likely in the event of (3), or indeed any blade contact with a solid object.
I've recently built my first electronically stabilised FBL heli. I can normally set a flybar up from scratch in about half-an-hour. The FBL unit took me nearly all day, with constant references to the user manual to set it up properly on the computer. I accept that this was partly due to my unfamiliarity with it, and partly that it is known to be a complex unit to set up! Not all units are that tricky. From a flying perspective, you need to be MUCH more careful with take-offs and landings. I manageed to strip all my servo gears (metal ones!) due to a slightly cack-handed take off, despite there being no other damage to the model or blades! Once in the air, there is less "sneeze factor" than with a flybar - but the same is true (more so, in fact) of mechanical FBL, which I've been flying for years.
So its horses for courses. If you are into full on 3D or FAI flying, FBL is the way to go. If you are a sport flyer, its a lot of money and hassle for little gain.
Multi-blade heads:
Again, there are two types: Those intended for scale, and those for aerobatics. Scale ones are often fully articulated (as per full-size), and as such, don't always need an FBL unit. Aerobatic ones tend to be rigid, and unless very big and turbine powered, will almost certainly need an fbl unit to make them flyable. I'm assuming the OP is talking about aerobatic use again.
Multi-blade heads tend to have more inertia than two bladers, which makes autos better. They also seem to run smoother. - a bit like the difference between four, six and eight cylinder car engines! Odd numbers of blades seem to work better than evens, many full size machines having an odd number of blades. Jim Morley used to say that he could never get his 4-blader to fly as well as his 3-blader, despite them being otherwise identical! Tha manufacturers would have you believe that a multi-blade head will respond quicker, but the maths doesn't seem to support that. The "following rate" seems to be dictated mainly by diameter, weight and rpm, rather than the number of blades. The improved response time - if it exists - must be down to other factors. However, the added smoothness alone is a big bonus.
They are, however, a pig to get tracking accurately! Its not simply a case of tweaking a blade and seeing if you make it better or worse, as with a 2-blade. You have to identify WHICH blade is out as well, and that is often a 2-man job! If you are lucky, and very careful with your initial bench set-up, you will probably get it pretty close immediately. However, with a bit of wear and tear, expect the tracking to start to go out, and that's when the nightmare begins!
Just my 2p worth! Other will probably disagree!
I have two tools in my tool box. A hammer...... and a condom. If that doesn't work then it's TJFGOP.... The Jobs Fd Going On The P:::::::
and a platinum star 
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