[janoskiss (Steve H)]

I know using a 2X barlow will double my focal length and 1/4 the amount of light available to view .

That's not really a "bad effect": it is the effect; you have a fixed amount of light and you're deciding how to distribute it across the focal plane. The more magnification the more thinly the light is spread.

However what I don't know is how this effect the view for visual work. I don't see it being a problem on the moon or bright planets however would it effect the visablility of faint stars in clusters or faint details of nebulas?

Again with visual the brightness goes down as magnification goes up (inverse square). Whether you are using a barlow to get there or a shorter FL EP, the effect is practically the same.

I find that with barlows the biggest issue is stray light and/or scatter. The Orion Deluxe 2" (aka UO 2") is a great barlow and the internal baffling works very well at minimising scatter and unwanted reflections. The only downside is that it is big, bit heavy, and needs to sit a fair way out of the focuser. (See Dave's review)

I would imagine it would effect these things or does our eye compensate for the difference making it less noticable?

The reason I ask is that I will soon be purchasing some EPs and I'm trying to figure out the best combo.

Like I already indicated, this is not really a question of using a barlow or not. It's more a question of what magnification for what objects. Two or three widefield EPs around the ~10mm (2mm exit pupil) mark has DSOs pretty well covered IMO. Powers much higher (exit pupil ~1mm or FL about 5mm and less) will have limited use on DSOs: just too dim.