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If there is a problem with a barlow it is usually one of the following:
1)Barlows increase the eye relief of eyepieces used in them. This is not a problem for most mid and high power eyepieces, but could be a problem for a low power eyepiece if the low power eyepiece already has so much eye relief it's difficult to hold your head steady enough to stay in its exit pupil at the long distance behind it.
2) Barlows often vignette lateral rays in an eyepiece that already uses the majority of the inside diameter of the barrel to field the light rays for that eyepiece. This most often occurs when barlowing low power eyepieces with field stops nearly the diameter of the barrel. This vignetting does not occur with mid- and high-power eyepieces. It is visible as a darkening of the edge of the field.
3) Some cheap barlows aren't adequately baffled and scatter light in their own barrels. Short barlows are not prone to this, but due to the deeper curves on their lenses, vignette more with long focal length eyepieces.
Despite the above, you should get one. The Orion Shorty Plus is excellent, as is the standard TeleVue.
Look for multi-coating on the lenses, and if 2", look for a free 1-1/4" adapter.
GSO makes an excellent 2" barlow.
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