I think Watchtower is pretty good, definitely one of the better arena maps. I've had some fun times on it. Congratulations on having it included as an official map, JoshuaC.
Still, I have to be honest and say that I think it's an odd choice by Valve, since the large open areas clearly go against their design philosophy and all that. I also wonder if the breakable glass is still there...
Junction is an ever stranger choice, considering all the times I've played it, Demomen ruled the entire map in the ultra-tight chokepoints. I don't even think Snipers will have anything to do on this map at all.