Wow, I'm surprised that I missed this thread up until now... this one is right up my alley. Christie and I actually covered this topic at the end of our Self-Defense Show Podcast episode 043.
http://www.SelfDefenseShow.com
Here's the thing with ANY self-defense device. It must be used in conjunction with a larger, more comprehensive self-defense portfolio. You should have in place all the prerequisite items and factors before using a device... such as a strong sense of self-worth, sensitivity to violation, the will to live, good (read: NOT most martial arts) self-defense training, awareness, de-escalation skills, victim unattractiveness, etc. etc. etc.
Also, bringing in a device (flashlight, knife, hand spike thing, gun, pepper spray, whatever) into a violent engagement really complicates the situation both tactically and legally. What if the device fails, or is taken away from you and used against you? That picture of the Surefire light with the over emphasized spiky bezel that's posted earlier in this thread would look really great on a huge screen in the court room, when YOU are being made out to be the bad guy to the jury by some slick tongue lawyer.
I'm merely brushing the surface of this topic here, because I'm too lazy to type all there is to say out.
I agree with GreenLED. Take yourself a FAST Defense or Model Mugging class FIRST. And then consider carrying a device.
For me personally, I carry several devices on me everyday. The one's I make public are...
A metal fist pack (no side spikes)... I tell people and others that it's for breaking a window in a car in case I'm trapped in a crash.
A Storm Whistle... no ball, works under water, loudest whistle in the world, no moving parts... did I mention it was very loud and will attract help / attention to a situation?
A Dereelight CL1H v4 R2 with a 1S driver
So long as all the prerequisites are met, and you understand the pros and cons of carrying a flashlight, it CAN be a useful dissuasion tool for semi-committed interviews (criminal dry run to test your boundaries).
If someone is approaching you in a parking lot (to be overly cliche), and they ask for the time, or bus fare to the hospital, or whatever... you could ask them to stop their approach. If they do, ask them, how can I help you... or whatever. (stop them as far away as you can) If they don't stop, they are obviously NOT respectful of you space / safety / wishes, so I would generally recommend this as the time it would be good to use your flashlight. Especially at night, if you put that beam in someone's eyes from a good distance (21 to 40+ feet) away, they will either stop and glare away, thus ending their approach, or they will have to charge in blindly (literally)... in which case you are at a physical tactical advantage. You have a good distance to respond, and they are charging in with their eyes closed or blinded.
Of course I recognize that this looks good on paper but in a real situation, someone may as well just pull out a handgun in response to your bright light... in which case, run like heck!
Anyways, that's my basic thoughts. It's up to you what you do in any given situation, so don't take my advice as pure guaranteed truth. Use your own judgement, and hopefully you will never have to face a situation like this.