Depends A LOT on where you live. If you live in a place that you only need to move snow 5-10 times a year, then a used one or even an El Cheapo unit would probably cut it.
However, if you live in a spot where moving snow is pretty much a daily part of life, then you have to pony up. My first year up here I thought I would be the "smart guy" and get a good used unit. Found an early 80's Toro in pretty good shape and have never been more frustrated with a piece of machinery in my life (except for maybe that RMK 700!)! Nothing worse than having to clear snow and not having a working thrower. It was a lot more than just a carb cleaning or adjustment that it needed (although it needed that too!). The list was long, but included things like a new friction wheel, new adjustment mechanism for the friction wheel, transfer case and gearing for the impeller.
The next season I listened to what locals had told me the year before and wedged open my wallet and bought a top of the line Ariens. As mentioned in a previous post, it has given me absolutely no problems since I got it.
So I guess it just depends on how reliable you need the unit to be. Sure you can save some money that can be used for snowmobile trips, but keep in mind if you are constantly busy fixing the thing, then you won't have time to spend that money snowmobiling! That was my case. Spent many a frustrating afternoon trying to fix it myself or at the local dealer getting it fixed and missed out on a lot of riding.
All things to consider. By the way, there looks to be a pretty good used Ariens for sale on this site. Check it out in the Misc. section of the classifieds.
-John