The most important post-SSE2 extensions are SSSE3 (pshufb) and SSE4.1 (rounding, min/max, blending, etc...). Pure SSE2 is a nightmare to use as it's a totally unbalanced SIMD ISA (a lot of missing stuff here are there requires a lot of workarounds and sometimes it's just better to go scalar). In addition, just [V]PSHUFB alone can do wonders and has a lot of application - I would say that almost all interesting problems can take the advantage of PSHUFB.