If you Shapechange into a Solar, you gain the following:
+18 Str
+10 Dex
+10 Con
Large size
Outsider creature type
Angel sub-type
Extraplanar sub-type (yes, you can be banished or blasphemed back to the solar's home plane!)
Good sub-type
+21 natural AC
Speed 50 ft (fly 150 ft good)
All Solar Special Qualities (too much to list easily)
You will lose any supernatural abilities of your original form. You will not gain spells or spell-like abilities. You will not gain their equipment (you will keep your own, adjusted to large size). Your HP will not change, except those granted by your Con bonus (and that will go away when you shift again).
Shapechanging into a creature with alignment descriptors give the shapechange spell those alignment descriptors. So an evil caster might suffer alignment consequences of changing into a solar (or be prohibited from doing it, at the DM's option).