I fell afoul of that issue. As regular bulbs started to run out I took to replacing them with energy savers. Ordered from Amazon. I was assuming a regularity in size and taking the lead from the plug, being screw-fit rather than bayonet-plug. Though the first order turned out to be a couple of bulbs like that pictured above, and while the plug might well fit it could never happen as the bulb itself was too wide for the casing around the light fitting.
Turns out the information I should have been looking at was a fitting code listed in the description, but without knowing it was there it just looked like extraneous letters. "ES" I think it was, being the regular size fitting for the regular bulbs. Armed with that info it was easy enough to find the energy saving bulbs for regular fittings.
After having looked into it, it also turns out that those bulbs that sit inside a enclosing fitting also need to include "reflector" in their description. I guess to ensure the heat produced doesn't knacker the fitting.
I can't say as I was satisfied with those describing themselves as "daylight" bulbs. I dare say the light was cleaner, less yellow, but not notably so. Found myself better sorted by a couple of fluorescent tubes in desklamps with long adjustable arms. More directional, and the light cast was noticibly cleaner.