With training, quality is in the eye of the beholder. Students might consider a class effective if they can find the solution to a problem that has plagued them at work or if they can learn something that helps them on the job. I know of students who consider any class that gets them out of the office for a week a success.
Employers tend to base their assessment of quality on how training affects productivity. However, measuring the productivity of network administrators and developers is difficult. Effective training might result in fewer mistakes, more effective preventive troubleshooting (which I call "stealth productivity" because you don't receive credit for preventing problems that never occur), and faster, more accurate responses to problems. Whether training is effective, therefore, depends on both your goals as a student and your employer’s goals as a company.
If you simply need to learn the mechanics of installing and maintaining a product, an instructor who knows just little more than you do can probably meet your goals. If you need to learn network-design best practices, an instructor who can explain the concepts from his or her own experience is a better choice. Many of the trainers who have been in the business for 5 years or longer have complained about the decrease in product experience and breadth of understanding that newer trainers bring to the classrooms; however, the reality is that just about anyone can show you how to create a new user account or set NTFS permissions. You, your employer, and your training provider must decide whether your goal is to improve your skills or expand your understanding of how a product works. That decision will determine not only which course you take but also which instructor teaches it.
Cost is another factor in determining the quality of training. I know of a company that paid more than $100,000 for its employees to gain access to hundreds of online courses. The company's 5000 employees receive all the training they want for about $50 per student per year—seemingly, a great deal. However, the quality of such programs is often lower than instructor-led training. The old adage "you get what you pay for" applies to training. If a particular training option is less expensive than all other options, consider what you might be trading for the savings.
Buying training, though, isn't like buying anything else. You must consider the intangibles, such as how well the courseware explains the concepts and enhances the learning process, how students learn and how much prior understanding they have, and how well an instructor can tailor a lecture to match the students’ learning styles. The intangibles determine training's overall effectiveness, and placing a monetary value on such factors is impossible. In the end, cost should be the last factor you consider—after you determine which training option can most effectively teach you what you want to learn.