This "triple play" by COMCAST is not Fibre a la Verizon FIOS. This is coax. The 6Mbps per second tier with Powerboost was, I thought, their lowest high speed tier available. If I were the OP - I would check what I was supposed to be on - and if it was the 6Mbps, I'd complain - you should be consistently at or near the 6Mbps mark.
The Digital Voice and HDTV - well that's your choice isn't it. Personally I have their HDTV and DVR and High Speed - but don't have Digital Voice. Really have no need for it - and am happy with what I got so...
I do consistently get speeds over 6Mbps - their powerboost is real, it's basically a best effort burst - so let's say you're downloading a nice big 10 MB file - it'll help tremendously. Downloading a nice big 100MB file - it'll help a little (they "throttle it down" after you use it for a bit).
The only question for you is whether or not you want Digital Voice - if you do - I'd say go for it, if not check on why your d/l is so slow.