I love my GSO 2" ED barlow (thanks again Kev!)
I agree with Kev though- I prefer not to add more glass to my eyepieces (it's also more mucking around in the dark, adding and taking off barlows when changing eyepieces). Therefore, I now keep the barlow for times when I can use very high magnifications when the seeing permits to boost up the mag. on my eyepieces of 10, 7 and 5 mm. Those times I will jump at the chance to use my barlow.
They don't happen very often though
. But still, I think it's handy to have a barlow in the kit for that reason.
I have to add though, for a beginner starting out, or for someone on a budget, it's a great way to double your eye pieces.