Interac has become the largest dispatch company for English teachers in Japan. Unfortunately, they have done this by cutting salaries, increasing the number of schools teachers have to visit, and providing little support.
Interac teachers generally are paid about 75% of their JET counterparts. In addition, Interac rarely helps teachers find an apartment (in JET, a furnished apartment is nearly always provided), may or may not offer health insurance (in JET, it is included), does not offer tax assistance (JET offers participants support with taxes), and refuses to pay you until one month after the month when you last worked (in JET and JET-like programs, generally you receive your first paycheck in the middle of the first month you work).
For these and other reasons, I can only recommend Interac to the most desperate people who can find no other way to Japan. Because their turnover rate is so high, it's generally easy to get a position with them, and they actually value Japanese ability (since if you can't speak Japanese, they have to help you more.)
The one positive thing I can say about Interac is that their contract periods are generally shorter (about 8 months), so if you just need to get a visa so that you can land in Japan and then start looking for other work, they could be a useful option.