Reasons to be bullish on the concept? Bitcoin is a secure public registry backed by a decentralized 300k THash/s computing network (and fast growing). That alone is insanely valuable, and not just as a vector of monetary transactions. It's one building block of our decentralized future.
Reasons to be bullish on the BTC/USD exchange price? That would be pure speculation. BTC seems to be now at low levels enough that it wouldn't be insane to buy some for short-term speculation, but it of course it would still be very risky.
The network is single-purpose and costs real electricity to run. If the mining cost stays above the value of the coins mined for long enough people will start turning off their miners.
The mining self-balances; if enough miners shut off their rigs, the difficulty goes down, and it becomes profitable (or break-even) to mine again. (Mining was heavily over-invested, and it's in the ugly process of rebalancing now.) It's possible that Bitcoin will be relegated to a fringe of techno-libertarians, but it is unlikely that the network will ever disappear.
Also, the network is "single-purpose" in the sense that an ethernet cable is single-purpose: while it only accomplishes one thing, it is a building block that can be leveraged in lots of interesting ways (smart contracts, proof-of-extistence, sidechains, colored coins, etc.)
Reasons to be bullish on the BTC/USD exchange price? That would be pure speculation. BTC seems to be now at low levels enough that it wouldn't be insane to buy some for short-term speculation, but it of course it would still be very risky.