When we first moved to this property a few years ago I converted 40 8 foot T12 fixtures to T5HO and could not be happier with that endeavor. After the utility company rebate I ended up at about $15 per fixture to convert them, bulbs included. Have not had to replace a single bulb yet on them.
For the rest of the building there is a combination of troffer and surface mount 4 foot T12 fixtures, about 80 bulbs in total.... I'm also looking into the LED tubes. I have not really looked much in depth yet though and will likely test with just a couple of fixtures before I pull the plug on a big pile of them. One thing I have decided on is I will be going with the more difficult ones versus the plug and pray ones. I just don't like the idea of having to continue to rely on (and power) the existing florescent ballasts so I'm going to go with a version of the tubes that requires removing the ballasts from the circuit (you could actually leave them installed in the fixtures and just electrically bypass them depending on what sort of retrofit you are looking into). There really seems to be 3 types.
The plug and play ones where the tube contains the LED driver electronics and requires that the existing florescent ballast remain in use. This would of course be the simplest and fastest option since its just like changing the bulb. One would also think this might be the most expensive option since it requires special electronics in the LED driver to convert the florescent ballast voltage to the voltage the LED tube needs.
The next option are also tubes with the LED drivers built in as part of each tube but this type expects standard 120v AC at the fixtures sockets (pin tombstones). Quite a bit more work to install these since you must disconnect the old florescent ballast and wire the incoming 120v directly to one bulb socket end per bulb. I'd assume slightly less expensive since 120v LED driver circuitry is so heaving mass produced that the costs for the components has to be a bit cheaper. Plus on this is you are no longer powering the florescent ballast (nor relying on it to keep working indefinitely).
The third option separates the LED driver circuitry from the tubes. This works much like your existing florescent fixture except instead of a florescent ballast, you replace that ballast with the LED driver. The actual tubes are just the LED's only with no extra circuitry inside of the tubes. This allows for a more reliable and robust LED driver section which will be installed under the middle cover right where the existing ballast was and is also protected from the heat of the LEDs (high power LED's DO produce quite a bit of heat and any good LED design will have a substantial heatsink directly on the back of the actual LED's) and replaceable as its own separate part if needed. This would also seemingly make the actual tubes much less expensive since no drivers are integrated into them. However the initial cost might be higher since you will need to get a more advanced LED driver that handles multiple tubes. The install would probably be slightly easier than the prior option though since you might not have to deal with the actual socket wiring at all simply swapping wire for wire from the old ballast to the new driver then install the new tubes.
This is a good read that shows the different options...
http://cltc.ucdavis.edu/sites/defau...Retrofit_Options_Linear_Fluorescent_FINAL.pdf
If I do any it will be the separate driver install. I just like the idea the best myself.