At 1920x1080 or 2560x1440 resolutions where Crossfire is fully supported, these cards will give better frame rates than an R9 290X or GTX 780 Ti in most situations.
Each card uses it's own VRAM, so even in Crossfire you effectively have 2GB of VRAM with these cards.
The limitation of 2GB of VRAM per card limits detail settings at higher resolutions, where the higher end cards have 3GB or 4GB of VRAM.
Even at 1920x1080 you can use more than 2GB in Skyrim with mods and MSAA enabled.
So if you are running 1920x1080 or 2560x1440 resolution and want higher frame rates, but are prepared to make some compromise on detail settings the Crossfire R9 270 cards may suit your needs.
Remember that your frame rate is limited by your monitor as well, usually 60 Hz (i.e. maximum 60 FPS).
Some games are also not well optimised for Crossfire and you may see micro stutter.
An AMD Radeon R9 290 or Nvidia GTX 780 will give more consistent performance and allow highest detail settings, even though this will be at a lower frame rate.