Nice!, just downloaded the beta and tried it out. Looks like it works (only tested the ad.asmx) and IsUserGroupMember will be real useful for me.
I did noticed that if you use "IsUserGroupMember" with a group that has a group you are in as a member it will show false.
I.E. : IT-MDT group has a list of members, one of those is IT-admin group which I am a member of. If i do IsUserGroupMember with my user name on the group IT-MDT it will say false