Many, but not all, 10-12' shrubs will end up 10-12' wide as well; how much room do you have between the side of the house and the property line? Will you want a walkway through that area? Probably, I would think. Do you want the hedge to stay in a straight line, more or less, or to stick out into the lawn every so often? Any plans for the back property line?

If there is or might be a problem with unwanted visits from next door a hedge won't be as effective as a fence. Unless closely sheared, to become unnaturally dense. But are you going to shear both sides - and the top - of a 12' formal hedge, maybe more than once per year? And how will you deal with going over and shearing the neighbor's side - this recurring project might in fact force you into come to their attention and interact with them, over and over, indefinitely.

Also something that shoots up that tall quickly enough to provide the desired relief isn't likely to stop at 10' - you really can't go shazam! and poof have a tall hedge in place in an instant, that stays a fixed height forever after. Naturally dense and tidy hedges like yew, holly or box that don't shoot out long wild growths are that way because they are slow growing - and may therefore require DECADES to grow 10' tall. Also a straight line of one kind of plant draws attention to an area, which sort of defeats the purpose.

Probably most often the best approach is a solid fence with an informal mixed planting of different kinds of trees and shrubs in front of it, including ones that will provide the desired height eventually, but will not be getting pruned to stay below a predetermined level.

Another way you can block a view is to block it where you are instead of where the view is, by planting, erecting or installing something right next to - or even on - the window, patio or other viewpoint from which you are most often bothered. This can be much cheaper and quicker than trying to cover the full length and height of the view with a planting.

Good points. I think a tree/hegde/fence is probably the proper combination. I have no desire to trim the top of the hedge, nor back.

I'm thinking fence, then make a long bed with trees every 15' or so. With shrubs/annuals under the trees.

Do you need the view blocked year-round or just in the summer?

Year round would be ideal. We have a large hill in the back of the development and the neighbors decide to plant crap on it. It's a rocky, nasty hillside, and every now and then you'll see them half way up. I'd like to really not look at them, but I have a feeling my luck is only good...

Better yet, maybe I move! Ha.

If you have the room, do a mixed shrub border with evergreen viburnums and conifers (and maybe some small trees). Give it 15-20 foot width.

Line of leylands spaced 4' oc 12 inches away from the property line. Haha oh wait that's what my neighbors did to my yard.

actually I shouldn't complain, yes they intrude into my yard but its better than staring at their house 24/7

How do the leylands look? I've heard those things can grow to enormous heights...