I haven't used any available drop-catching software and so I can't speak for any specific one, but the problem with drop-catching software is generally 3-fold:
1. It requires access to registrar APIs to perform well, and these APIs are difficult and often expensive to obtain access to. From my experience Dynadot's API has the lowest barrier to entry, but even DD requires that you first pre-deposit $500 for a bulk account and then continue to spend $500 per account per year. Yes, there are non-API drop-catchers available, but these are technically illegal to use and even if you're not caught, they perform very slow in practice. An, of course, you'll also have to pay money ($50-$100 usually) for the drop-catching software.
2. Even API drop catchers are slow and come with heavy restrictions. Dynadot only allows one REGISTER request even 7 seconds per account API key (each one requires a $500 pre-deposit), Moniker once every 60 seconds and technically Moniker strictly forbids using its API for drop-catching. Okay, fine, you'll save about $1 over a GoDaddy backorder on domains you do manage to grab, but GoDaddy backorders harness at least 8 registrars each sending dozens of REGISTER requests per seconds in attempt to secure your domain. You'll never beat GoDaddy's backorder system with a desktop drop catcher.
3. The majority of end-users I've dealt with use GoDaddy. So if you catch a domain with Dynadot or Moniker and attempt to flip it, the end-user will often demand that he/she transfer the domain to his/her GoDaddy account. Then you have to explain the whole 60-day transfer restriction and force the the end-user to wait -- allowing him/her to seek other alternatives, lose interest in your domain, etc. It's just so much easier to catch domains with GoDaddy and push the domain to you end-user's account when you sell it.
