P2Pool still worth mining?
Look at it this way: 


Everyone trying to solve a Bitcoin block is actually competing
against everyone else (I win you lose.)
Pools in general are essentially a pack of miners who collectively
agree not to compete with each other, but agree to collectively
compete against everyone else who is not in the pool.  Pools grew in
popularity as the difficulty grew and theoretically it has some advantages.
p2pool, of course, follows this same model, except now that the difficulty of obtaining a p2pool share is so difficult, small hash rates have become obsolete in the p2pool model
The p2pool model is a great idea and is still viable for anyone with at least 1 ths, but the model needs an update:
The share chains needs to be larger, the share rate needs to increase making the difficulty go down.  This will allow even the small miner to chip in.  Every hash counts.