Jim,
I would not change the lamp in a vent hood unless I were certain it was not going to get too hot. CFLs hate getting hot.
Your question, do CFLs really save noticeably:
You will notice the change immediately, however, just how long they take to pay for themselves depends on how much you pay per kilowatt energy and whether you got the bulbs rebated.
Here in Munich, one Kilowatt costs me .19 Eurocent. Lets make it .20 for ease in calculation.
Incandescents:
5x60=300W.
That means, if you run the bulbs a bit over three and a half-hours per day, your cost will be .20.
Do that 30 days in a month, that makes 6.00Euros.
OK, let's see. I can buy a decent CFL for 5.00Euros, but I can buy six at a time in the same good quality for 20Euros.
How long until I have saved enough money to pay for the six new bulbs?
Well, let's see.
6 x 9w =54 W (I know, I know, but I like the slightly less efficient Philips bulbs which give better color rendition. figure 7W/bulb if you like)
54w x 3.3 hours =178.2 Watts.
178.2 x 30 days = 5.346 Kw
5.346 x .20 = 1.07 Euros
Cost to run incandescents: 6.00 Euros/month
Cost to run CFLs: 1.07 Euros/month
Savings: 4.93 Euros/month
So, in shortly over 4 months, the CFLs have paid for themselves.
Their life expectancy is longer than an incandescent lamp, so I left that out of the calculation, but even if you are getting your incandescent lamps for free, the savings are real and you will see them at once.
Of course, those electric heaters are what are really costing you.
1.5Kw x .20 = .30 Eurocents/hour
Yikes.
Hope that helps.