iris is an alternate client for uo,
displaying uo in 3d is one big point, but there are others as well,
for example it can be compiled native on linux,
there is a 2d mode with transparent water and smoother terrain,
on some computers with good hardware it can run more fluid than the original client,
it has some interesting options for scripting and better syntax than the easyuo scripting language,
the windowsize can be changed freely,
more space in the backpack to arrange things,
the map in the compass shows creatures and can be zoomed,
and it might even be interesting for free-shard creators, as the original client has certain limitation whereas iris can be customizable and extended.
that being said, i'd say it is can still a bit difficult for the average user to play with iris,
for example due to missing 3d models, hotkeys having to be defined by editing a textfile as there is no graphical ingame dialog to set them yet, and a bunch of performance problems, especially in large cities, where we still have to do lots of optimization.
if you want to get an idea of how uo looks in 3d, i'd sugggest taking a look at this youtube vid :
http://iris2.de/index.php/YouTubeTrailer08and maybe our screenshots section :
http://iris2.de/index.php/ScreenshotsIris itself should run on most shards, if you have problems on a particular shard, report it here in the forums, and we'll try to solve it.
Iris 3d mode might not make sense on shards with lots of custom graphics for which there are no 3d models yet, but iris also has a 2d mode.