First things, first, I don't think that Twiends is a scam - I can see no evidence to suggest this. They have never updated my twitter feed and have provided me with followers. Though I have never used the paid version. When you read their about section you see how they are trying to operate within Twitters Terms and Conditions.
Twiends Explained
The basic concept behind Twiends is pretty good. and very simple. You follow someone in return for so many "seeds" (usually between 2 and 9). The seeds that you gain can be used to gain followers. I was very impressed with how fast my follower count grew.
Drawbacks
However, there are major drawbacks with Twiends. The main one is that people follow and then unfollow you so you have wasted seeds. In addition people who use Twiends tend to have be following huge numbers of people so don't make very good followers.
However
Despite this there are advantages to simply having a high number of followers even though most of your followers are following so many people they hardly notice you. For example these crap followers do generally retweet a lot so as your follower numbers increase your retweet rate increase. Also having a large following gives you, strangely, credibility. People look at you as being influential and known whereas what you are really doing is just trying to punch above you weight.
Overall
I would say that using Twiends is worthwhile. For a start you can get 10 seeds (at most 5 followers) simply for logging in so if nothing else this is worth doing. It is a good way to get started and let people know you have arrived on the Twitter scene.