My advice is to steer well clear of them.
Using services that guarantee you reach the Digg front page for $100 or even $40 if you look hard enough on certain webmaster forums is a huge risk.
If you are a large brand and get caught out using these services the backlash can be extremely damaging.
Do you want to place your brand equity in the hands of somebody who Diggs stories for a living?
One of the main drawbacks to these services is that they often fail, not due to the lack of votes but because they let poor content into their systems.
The owners are so desperate to make money they allow advertisers to submit stories that are blatantly spam and have no chance of making the Digg homepage, let alone staying there for more than a few minutes.
This willingness to accept low quality stories means the systems are liable to come crashing down at any time.
If you happen to be involved with the system when it gets "outed" your brand will be dragged down with it.
The best way to ensure social media success is to learn what people want to read on sites like Digg and StumbleUpon by becoming a user of these sites yourself.
Once you know what stories are being made popular every day go out and create more of the same.
Build a core audience of tech savvy social bookmarking users and your content will eventually become popular. But only if it's good enough.
My best tip for getting on the Digg homepage is to test the content with StumbleUpon users first.
If you start getting thousands of visitors from StumbleUpon then some of these will vote for your article on Digg, giving you a nice headstart.
Related research: Online PR Roundtable Output