Well, I have a Barnett Commando crossbow, but one fitted with a 220lb crossbar rather than the standard 150lb. All I can say is take GREAT care where you use it, as I've tested mine at our outdoor range and it'll put an aluminium bolt over 2" into a tree, and up to 200m distance if fired at 45°.
Saying that though it is great fun to use, with a dead easy cocking mechanism (the stock pivots away from the bow pulling the string with it) and an automatic safety. It's also compact and rather light, although the trigger pull is pretty severe (get a trigger shoe, makes life much easier).
As for a scope... Well, I did try putting one on my bow, but it rattled off after a couple of shots. You CAN use a scope, just make sure you use strong double-screw mounts and arrester pin to hold it in place. Also you'd better use a robust scope (maybe a WTC 1.5-5 x 20?), the kick off the string is more than you'll get from most spring rifles. To be honest though, maybe just a good red-dot sight might be better.
I hope that's of some help.
To reiterate though, if you do go for a full-size crossbow with a 150lb string (or there abouts) then take great care if you use it in your garden, my recommendation though would be not to. Wait until you're on open ground (preferably an outdoor range or archery club). If you want something to use in the garden try one of Barnett's 50lb pistol crossbows and save the daddy for the fields.
Trust me on the power though, I fired mine in the back garden at a piece of wooden decking (approx 1.5" thick solid wood) with a thick chipboard backstop behind it... The bolt went through them all and still had enough energy to leave a hole in the garage wall as well...
This is SWAT <img src="http://jsramsbottom.co.uk/bbs/e/icon_cool.gif" alt="Cool" width="15" height="15"><!--graemlin:--> Strachan, signing off.