Many issues here. Let me go over in as much detail as possible:
Background: I love energy efficiency, and when doing my kitchen researched this LED Vs CFL vs incandescent lights extensively. Notice there are also permit (legal) implications, depending on which state you leave (CA has Title 24 for example).
Connector types: There are several connectors that make the decision of which light you can use tricky. the main ones are
Edison: The screw type, accepts CFL, Incandescent and some LED bulbs
Pins (2 or 4): Accepts florescent. Was invented mostly so that for permits purposes, they can make sure you are installing an efficient light and will not go and replace the CFL bulb with an incandescent one after you pass the permit.
GU10: mostly used for high voltage halogen
GU5.3 Bi- Pin: mostly used for low voltage halogen
High level rules:
You never go with incandescent when it is incandescent Vs CFL (Edison, screw system). LEDs are getting there, but not enough to justify the price and their light output is very low.
You can only go with pin florescent if you have a florescent "hw". that is the only thing that you can plug there by design.
Never use Halogen. If it is a GU10 (line voltage), get a $20-$30 LED (MR16). you get 4W Vs 35W. Light output is lower, but you can get HW with simply more heads. Replace 3 halogen with 5 LEDs and you are fine in terms of light and energy savings
If the halogen is a low voltage, things get tricky. You can just replace the head with a 3-5W bi-pin LED. they run $10-$20. They problem is that you need to make sure your low voltage converter will work. converters have what's called "minimal load". For most of them, they expect a 35-50W halogen so they have a minimal load of 10-20. That means that when you plug a 3W LED, they may not kick in. you solve it by either using a remote transformer that feeds "enough" LED bulbs, or get a transformer that is "led" designed, but really all it means is that it has a minimal load of 3w and goes all the way up to something (I have a 3-60W for mono-points).
Lat general tip: do not mess with dimmers. Just on/off.
My personal choices:
For kitchen recessed light I got LED LR6-GU24. It is 12W, think that the florescent way would suggest I put a 23W bulb for each of these (I have 7). That means 50% saving. Same light output (they say it is 65W equivalent, but it feels like closer to 90W). No one can tell it is LED BTW, it is bright and even (not a "pin" of light, and well spread).
Cost efficient: great! less energy, and upfront cost was the same as getting a florescent can combination. (you may get cheaper florescent if it is not for a permit and you do the Edison type)
For a bat lights I went with 3 mono points. Each has a transformer built in. I did not (suggest you never) buy "LED" lamps since you pay a huge premium for getting the LED HW. I just got the transformer with minimal load of 3W. I then threw away the GU5.3 Bi- Pin halogen and got Maxxima LED MR16. They are very good price (value) company. I compared the light to a $30 GE and these one are just as good. Notice that at 3W it is indeed just ambient light. good for over the bar in addition to some other light in the room. If you need more power, get 4 or 5W bulbs. see maxxima site
For spotlight lights in the center of medium size room, I went with much cheaper "line voltage" fixture from Lowes. These fixtures expect GU10 halogen (25-50W). I got in this case a the more expensive $30 LED GU10 bulbs (GE, at home depot). They are great, but I hope you can do better on the price. I have 5 and for medium size dining room, correctly targeted, it is nice light to use while eating.
Best Answer
Yes, though not always. Track fittings in particular use many bulbs (usually 12V MR16) off a single transformer/driver.
The transformer may have a minimum load, below which it won't switch on or will oscillate between on and off. Have you tried putting the old halogens back in? If they work, the transformer is OK.
Nope.
If you want LED bulbs (and don't mind about dimmability), it may be easiest to replace the MR16 downlights with mains-voltage GU10 ones, and use GU10 bulbs. No transformer/driver required.