This has been discussed in this thread:
http://www.yosemite.ca.us/forum/viewtopic.php?t=560
And yes, I highly recommend it. I really like this part of the Park. Most people only go to Kibbie. If you are willing to go cross country, you can have some of the best solitude in the park.
I had planned to go out Styx Pass towards Mercur (as far as the snow would allow us) a few weeks back, but overtime at work came in the way, unfortunately. However, you shouldn't have anything to worry about snow wise at the end of May early June.
Bill-e-g has some excellent knowledge of that area.
I have been to Little Bear, and I wasn't super impressed. Boundary was nicer, but there was only one decent campsite at Boundary. The odds are that you will most likely have it to yourself. Although, the time I was there, I accidentally interrupted a backcountry tryst between two rangers. I haven't been to Spotted Fawn, so I can't comment on it.
The way I did was: in to Kibbie the first day (it was off working a graveyard with no sleep for over 24 hours, so not much progress the 1st day). Up the ridge onto Flora the 2nd day. From Flora, cross country to Boundary the 3rd. Then out Styx Pass back to the parking lot the 4th day. It may be difficult going cross country when the water is flowing early in the season and getting from Flora to Boundary though...
However, if your destination is Spotted Fawn/Little Bear, it would be a good deal easier to go in the Sachse Springs to Styx Pass route to Boundary Lake. It is probably more miles in, but its on a pretty decent trail and you'd make better time.