If you are looking for a nice 10-speed rear derailleur for a build or to replace an old Shimano derailleur, the R10 is a great option. I used the Microshift 10 speed group on my Surly build and it has worked great all summer. Check it out:
Features
Forged and CNC machined construction
Lightweight Aluminum
Anodized black or Polished silver colors
Short, medium, and long cage options
Compatible with Shimano
Specs Short Cage
10-speed
Max Capacity: 33T(short)
Max cog: 25-28T (short)
Steel Cage
Weight: 216 grams
Installation
Installation is really easy if you are familiar with installing Shimano as everything is about the same. Here are the instructions from Microshift:
Great replacement for old Shimano 10 speed derailleurs
If you have a tired old Shimano derailleur, this is a great replacement. It even comes in silver so you can maintain the same look. Microshift also has 8 and 9 speed options.
Frugal Average Bicyclist 
I imagine some of this older Shimano stuff is hard to find, and those who have it are either giving it away or asking for a lot of cash. Looks like you got a good deal.
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Thanks, I think so.
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