Is it worth buying a x2 Barlow - Definitely

[bingevader]

If you have, say, 5, 10, 20 and 30mm, what do you gain from a Barlow? After a few years mine fell into disuse and I'm not even sure I could find it now!

 Or similarly to Steve's example, what about a 30, 24, 8 and a 2x Barlow?

I took just the 24, 8 and barlow on holiday last summer, fine for a fortnight away with only a couple of nights observing.

[Charic]

As already mentioned by FLO, having the correct EP focal lengths in the first place, is essential. Otherwise nothing is gained, except maybe some eye-relief.

When I started  buying my EP's.  whether by luck, fortune, or just  convenient, I've noted since buying the first 8mm BST and a previous Revelation Barlow, that the BST Starguiders can all be Barlowed, with no duplication of magnification, so a good start for my needs.  

Also when Barlowing an 8mm EP, remember it does not become a 4mm EP!  It has the same power maybe, but its still an 8mm working with a larger focal length from the telescope, afforded by whatever factor the Barlow is providing?

[FLO]

... the BST Starguiders can all be Barlowed, with no duplication of magnification, 

I know you are a huge fan of BSTs but that is common practice amongst eyepiece manufacturers, focal lengths are often selected with a barlow in mind. 

Also when Barlowing an 8mm EP, remember it does not become a 4mm EP!  It has the same power maybe, but its still an 8mm working with a larger focal length from the telescope, afforded by whatever factor the Barlow is providing?

Yes, a 2x barlow would provide the magnification of a 4mm but the eye-relief of an 8mm (a regular barlow actually increases eye-relief, ever so slightly).

HTH 

[FLO]

This might surprise you Charic, the BST series design almost certainly incorporates a barlowing component. Particularly the shorter focal lengths. The 3.2mm, for example, probably has a 4-element component with a native focal length around 6 or 7mm and a 2-element barlowing component, around 2x, to achieve 3.2mm without too much loss of eye-relief. This too is common practice. 

Steve