Having taken the Trinity TESOL, I can give a fair review.

Trinity College London Certificate in TESOL (Teaching English to Speakers of Other Languages) is an internationally recognized professional teaching qualification. It is designed for people who want to enter the field of Teaching English as a Foreign Language (TEFL) or TESOL, as well as for practicing teachers who want to update their teaching methodology and style and gain a TESOL/TEFL qualification. The 4-week introductory teacher training course is conducted 2 – 3 times per year at several locations world-wide.

 I enjoyed the course and learned a lot. At the time(2006), the trainers at IALF, Bali were highly experienced, the class size was 10, the fee was "usual", and the facilities/resources were spectacular. My diploma has been recognized at language schools in both Thailand and Indonesia with no questions asked. When I started my first English teaching job, I was well prepared and succeeded at my job thanks to the training in this course.

On the downside, I've found it's only a preliminary qualification and continuing education has very limited options. You basically have to go for a DipTESOL next which is offered in very few locations like Ireland, etc. Also your home country won't give you much for your troubles other than a few credits in English.

 All told, it's worth it if you want to try out teaching before you commit to something more involved. If you have already decided to commit, you'd be better off with a Degree and a continuing education plan.