The answer to how secure is bitlocker is that if someone has physical access to your PC, it depends on what YOU will do next:
you won't access it ever again (PC stolen, seized, whatever), then unless they are a governmental entity, they cannot access your data, now. If they keep the disk for a few years, chances are some flaw/bug/vulnerability will have been uncovered by then and they will be able to access your data with ready-made tools and no expertise needed. But maybe there are no bugs and no flaws...
If you will use the PC again, then I hope you have a strong BIOS password, and confidence in that BIOS, because otherwise they could just flash your BIOS with a keylogging one.
you're just off for coffee, just a minute... then they could just wipe your keyboard real clean, wait until you enter your password again, and then distract you while they take a snap of your keys. The dirty keys will be those of your password, even if you entered a few extra keystrokes, this info greatly diminishes the complexity of brute-forcing your password. Also works with UV glowing powder, cheap "invisible" ink, etc.
So basically Bitlocker will stop a casual thief from accessing the drive of your stolen Laptop, but it won't do squat against a determined adversary that has physical access to your machine while you are away.
And note the last two apply to all encryptions that only involve only a password, as well as greatly diminishing the security of smartcard/PIN-only systems if they can steal the smartcard.