Tenpoint makes really good scopes that come on their bows. I have the Rangemaster Pro on one crossbow and and the Evo X scope on my.other crossbow. Hawke also makes really good scopes also.
I will second Rakkin6's statements here.
I was in the process of buying Hawke's top crossbow scope when a friend showed me the Tenpoint Rangemaster Pro. These two are more similar than different, but the Tenpoint Rangemaster Pro is less confusing to use, with easier to see aiming points.
MOST "crossbow" designated scopes are low-quality glass, i.e. "cheap" crap.
Some of the worst have been Excalibur's,
never mind I'm a big fan of Excalibur's crossbows, just not their scopes.
I was given Excalibur's latest top-of-the line crossbow scope, and it does appear to be decent.
(I guess they just wanted me to stop saying bad things about their scopes!)
Currently, I am using three different crossbow scopes.
1) A 1.5-4.5x Nikon Monarch "shotgun" scope.
This is on a "back-up" crossbow, and placed in use when I simply couldn't find a decent "crossbow" scope. When I get around to it, will probably replace this with the Excalibur scope was given, and then put this "shotgun" scope back on a shotgun.
2) Tenpoint Rangemaster Pro ---- actually love this "crossbow" scope.
3) Burris Oracle X --- the one I most often use
I mainly got this because it's was the only "crossbow" scope I could find with 2x magnification (i.e. wide field of view), that could also be cranked up to 7x without changing POI while using the same aiming points. It also has a built-in rangefinder. I did not get this scope for long-range shooting. I got it because I wanted a wide field of view and a built-in rangefinder.
On top of being "junk", most crossbow scopes have too much magnification for shooting at an average distance of somewhere around 20 - 25 yds.
How much of any deer can you "see" at 15-20 yds when your rifle scope is set on 4x or higher magnification? Magnification of @ 4x to 5x is where most "crossbow" scopes get set. That is more a liability than an asset. The way most "crossbow" scopes work, the higher your arrow (or bolt) velocity, the more magnification you must use (to make the aim points work correctly). It's akin to placing a fixed 12x scope on a 30-30 lever action rifle for shooting at deer under 100 yds.