I like sending and receiving them. Sometimes the decisionm process is so fast that there is hardly time.
One thing I always have done, however, is to write to the companies I interviewed with but that I decided not to pursue (whether I had received an offer or not.) In my opinion, this accomplishes two things: First, it allows them to take my application out of consideration. Second, it tends to leave a good impression (assuming the letter doesn't say something like "Forget about the job offer - I think your company stinks" <grin> in case at some point in the future something comes up at that company that really interests you.
I actually did what Peter mentions above.....after an interview couple months back I realized a bit later "you know what.....you're going to be really bored after a year or so because you'll mainly be operating in 'maintenance' mode which for me is kinda boring. So, I got in touch with HR and proposed they hire me on a 1-1.5 year contract because that was the duration I felt would be needed to meet some of their goals that were communicated to me during the interview.
In regards to the thank-you notes I've received by various people over the years which has been few and far between the verbage that's used has generally been so generic and seemingly fake I never pay too much attention to the letters.
I approach the entire interviewing process from the perspective of being "real". If you're in the habit of meeting new people and sending them thank-you notes about how much you enjoyed meeting them...go for it. If you normally wear a suit to work each day...go for it. If this isn't 'you' don't worry about acting in a manner to give a somewhat false impression of yourself and how you operate or worry that you're not falling into the status quo via how you as a person operate as well as what defines you as a person. Within reason of course....I made the choice to only put in blond highlights a few weeks ago and skip adding red which I normally do because I'm currently in interviewing mode. Annoys the heck out of me but whatever.....I know there's many who'd immediately have ridiculous preconceived notions about someone who puts in red and gold highlights into his hair. I personally have no desire to work for such a person who judges people's ability to perform by their hair color, write thank-you notes and what they decided to wear to the interview.
I made it through interviews with the CEO, CTO, VP of development, VP of product management wearing jeans and got a call from the CEO afterwards they liked me and we'd starting talking salary in a day or two. One's choice of clothes was something they hadn't found from their experiences affects performance.
Although I am broadening the scope of the original question this is how I approach things....break norms of society when it makes sense. If you've got the skills and are able to communicate how such skills will help a company have the guts to walk into the interview as if it were a typical day at the office.
Reserve a few months every so often and preview retirement throughout your career. You won't regret that a 35 year career was reduced to 34 years to take vacations measured in months in order to remember what a stress and care-free life is all about.
Books and hard work will get you anywhere you want to go.