For my last couple of games, I've actually played them and theory translates into function even better then then I thought.
First, bikes are cheap. They dropped 7 pts each from the previous book. For that, they are T5, WS4, S4 on the charge with 3 base attacks.
Second, they are really survivable. The aforementioned T5, a 4+ armor save and thanks to the new "exhaust cloud" rule they have a 3+ cover save when jinking.
Third, they still have great firepower with 3 twin-linked S5 AP5 shots each with a 30" threat range. They can be a threat to most flyers with that..
Forth, they are FAST. Just like any bike the 24" a turn movement means you're in your opponents face right away. Did I mention they have 'ere we go, meaning they get to re-roll one charge die? On a BIKE!?!? They are cheap enough and hardy enough to really be the hammer in the hammer and anvil. Boost them across the board while marching your infantry behind them. Your opponent will be far to busy dumping firepower into the bikes to worry about anything else..
Last, you can really buff them up. Now that you can take a painboy as a character, and give him a bike, giving the bike unit FNP is really amazing.
If you really want to go crazy give them a biker boss with Da Lucky Stikk for WS5 and LD9. It will also allow the biker boss to tank those S10 shots!
The one downside is the low leadership. Once they drop below 10 models it isn't hard for them to run away even with mob rule. However if you have at least 10 models they won't run, as the S4 hits probably won't do very much with T5, 4+ armor and FNP.
So are they the best unit in the Codex? I'm not sure, but it's hard to ignore all that speed, firepower, hitting power and survivability.