I use my Taig mill a lot and get very good results with 1/4 inch end mills, but I do slow the plunge rate down a lot. The Taig doesn't really like anything much larger than 1/4. Have used 5/16 with some success, but a 3/8 will really bog it down.
Best results have been achieved using high helix center-cutting carbide tools. They cut 1/8 to 1/4 inch deep in one pass (6061 aluminum) without a problem at a speed of F5. Have not pushed any harder than that. Generally requires little or no polishing for a really good surface finish.
Bob@BobsShop