It might depend on your age and your personality type but I like having structure at my job. Having people tell me what to do in a concrete way is a good thing. When I was an ALT in Fukushimacho in Hokkaido I had nothing like that. I mostly taught at elementary schools and was told I could do whatever I want. I don't think I did a bad job but I think I could have done more had there actually been someone overlooking me and telling me how to use the the text book, software and various other things.
Where I am currently working is completely different. I get emails almost every day from various schools telling me what we will be studying next. That makes being prepared extremely easy. Also many of the teachers are extremely active in the classroom making it more of a team teaching environment, which can be good and not so great depending on the teacher.
Interac was quite a while ago now but I still remember the feeling of working for them. You only meet with the people who manage you a few times a year but whenever there is a "complaint" about your teaching, you will get an email that pretty much always has a dark undertone to it. I was working in a large city in Chiba and there were so many schools and classes it was really hard to manage and since it was my first time teaching it was a bit overwhelming.
There are many other companies besides Interac that are very similar like Heart English, Altia and many others. They are usually quite easy to get hired by but that is because they have a pretty high turn-over rate. The pay is always less because they are essentially a middle man between you and a board of education. You'll usually get more vacation during winter and summer but you also will get a fraction of your normal salary during that time.
Being a direct hire is the closest I've had to being treated like a normal Japanese teacher. I have 10 paid holidays a year. I have to use my inkan to stamp in and out every day. I have to report to the board of education every day. There are also several projects throughout the year that are mine and the two other ALT's responsibility. It may sound rough but finding an ALT job that pays as much as JET is really hard.
Most non-JET ALT jobs will pay between 250,000 and 280,000 yen. Also you will have to pay for your own apartment. In my experience in Japan and decent apartment will run you around 50,000 yen every month. I have heard of direct hire ALTs in remote areas being provided with housing but it is a blessing and a curse. It usually means you be in the middle of nowhere and you will have a 30 to 40 year old toilet and bathroom. This would be fine in the USA but once you are used to the comforts of modern Japanese bathrooms switching back is extremely difficult.
I got lucky in many respects. I earn 300,000 a month and my apartment is 40,000 a month which includes parking and internet. There are many people in JET who make less than me also. I have heard of people in large cities like Kyoto who have to pay for their own apartments and they can be up to 70,000 yen a month and the are given no extra money from their board of education.
When I first started here I was talking with one of the other ALTs and basically asking him why everything was a bit more strict here. Apparently the ALT who I replaced had been there for quite a long time and decided that he didn't want any special treatment. He wanted to be treated just like any other Japanese employee would. I respect the guy for that but at the same time, having more than 10 vacation days in a year would be great.