Are guerrilla marketing techniques actually effective?

Yes they actually work and there are many studies that show evidence of them working.  Read Jay Conrad Levinson's books or the many papers on the Internet that are proof of them.  One of my "old" favorites is the Marlboro man. Every time you create buzz (just like viral videos) that is Guerrilla Marketing at work.

Yes, they are very effective but you have to take into consideration what they are doing for your brand. Most guerrilla tactics are all about branding and generating awareness of your company or product offering. Sometimes what you get most of is "buzz" or recognition and it doesn't necessarily translate into higher sales. You have to view marketing tactics as two-fold: branding and lead generation. Guerrilla marketing usually works the most for branding.

We did one few weeks back that successfully for us. http://relatas.com/ is a product start-up and we had our beta product launch a few weeks back.  I wrote a detailed blog on how we successfully executed guerrilla marketing at a national event. http://blog.relatas.com/guerrilla-marketing-for-startups/ . Feedback welcome and would love to hear how others are doing.