yeah, I've had one stolen myself, luckily I only spent $230 on the whole thing. My current one has almost $1000 dumped into it (including spare and aftermarket parts). Bike locks will only slow a thief down, if they KNOW they have the time to get it, they will, but most thieves wont take the risk and move along. I highly recommend against cable locks, as I was using one when my first bike was stolen. Simple bolt cutters get through them like butter. Chain and locks are the next step up, obviously harder to cut, probably much noisier too, most thieves wont go for this and the next level up: the U lock. My lock is rated to 40 Tons, basically enough that if someone attempts to remove the lock using a truck, it'll either destroy the bike, or the truck hitch/frame/whatever they used. Cheap U-locks are susceptible to drills so get yourself a good double-locking one. I use bullguard, they have a $2,500 guarantee that your bike is safe (it may have been $10,000, I dont really know. This basically means if your bike is stolen, they'll cover a replacement up to said guarantee).
And the final level, as stated if a thief really wants your bike, they'll get it. Thats where spybike comes in. How are they going to know the seat post is a tracker? They gunna ride it without the seat? enjoy the bumps
. Anyway the spybike or phone tracker route is the only way to 99.9% guarantee that you will have your bike as long as you want it. Noone is going to check for a GPS or well hidden phone (has to be attached to the bike, they could very well just rip bags off) when they're in a hurry to get away with it.