I've used Rentacoder several times and have had mostly good experiences. These are my takeaways:

- Make sure the project deadlines are well-defined. Coders have a tendency to over-promise and therefore projects can drag on and on unless you set strict deadlines and stick to them. Rentacoder's dispute resolution system works well so don't be afraid to use it.

- Complicated projects are hard to manage remotely. Small, simple projects work best on Rentacoder.

- Rentacoder ratings are generally correct. If someone is highly rated, they are usually capable and responsible.

- Prices are generally cheap on most projects so it can make sense to give bonuses for extra work or to incent your coder to finish more quickly.

- Rentacoder prices are generally a lot lower than using a software development firm and can even be lower than using an India-based firm since coders are typically bidding as independent contractors.

- Try to find coders that have had experience doing precisely what you are looking for (you can read this in their profiles).

Hope this is helpful.