Vantaggi
I've worked as a supply teacher for TeacherActive in Nottingham for the last 7 years. This is the best agency I've worked for: the staff are friendly, professional, and work really hard to get their supply workers into good positions. The best part for me is that they will work around your needs as an agency worker and get you work that you're happy with. I have a chronic illness that affects how much I can work, and if I didn't have the flexibility working for TeacherActive offers, I don't think I would have been able to stay in teaching at all. If you want long term placements, the team will work hard to find you those. If you need more flexibility, they'll work around that. I have recommended to many, many colleagues fed up with unprofessional agencies or poorly managed schools to try TeacherActive and those who have taken up my recommendation have always found it worthwhile.
Svantaggi
I don't really think there are any. The downsides apply to supply teaching as a whole and TeacherActive works well to mitigate them (for example, not getting exactly the amount of work you want or having a challenging class).