Mine can raise a tank to 30c from 15 in about 6 hours, when it took an internal of the same power 3 days....
They will draw the same power as an internal when on, but they won't be on for as long when heating the tank for the first time, thus making them cheaper. They are more efficient from a heat-spread point of view, because they are in-line and always have good flow through them if you keep the filter clean
On a side note though, once the tank is on temperature, I don't realy notice a difference in "on" time... The in-line goes on and off more often the the old internal did, but they are ultimately on for about the same time as each other...
Internals tend to be placed out-of-sight, so they are usualy in an area of low flow. This caurses a heat bubble to form arround them, that takes time to spread through the tank. All the time the heater is off, because it's bubble is at the right temperature and waiting to be spread into the rest of the tank, the tank is cooling while still not quite at temperature.
If you waterchange direct from the cold tap, as most with large tanks do, you will see a big difference in the heat-costs of a waterchange, and the temperature won't hang just below the set one for a long while, so overall in that way they are better. If you re-fill with temperature matched water, you probibly won't see any difference
On a side note though, I haven't heard that many issues with the Fluval FX5 yet. Whether that is because it's just new I don't know, but Fluval may have fixed the reliability issues at last... I got shocked by my old 404 after it's motor unit packed up, and it was a 404 that drained my friends tank for him unexpectedly via the supposidly waterproof main seal ring... I also know of a few 05 owners through work, whom have had the quick-release valve go on them, leaking quite badly... If you are after something like an FX5 for a large tank, go for it is what I'd say, but any 05 model for tanks upto 300l, I'd forget about them and go Eheim or Tetratec
All the best
Rabbut