Are you looking for awesome sound, or awesome sound with additional power. If you are just in it for the sound, I would suggest the OEM exhaust mod (Perforated pipe replacement method)
If you want to lose some weight (mostly in your wallet) and gain a bit of performance, then get the Eisenmann or Akra. I went with the $500.00 option and I'm very happy with it. (I purchased a used unit for $150 instead of hacking my new OEM exhaust or it would have been just $350.00)
2011 Sapphire Black/Black E70 X5 M "Torquey"
2010 Jerez Black/Black E90 M3 DCT -Gone-
2009 Montego Blue/Grey E90 335i -Gone-