I always wondered what those little 'seahorse' things on the Termagant sprue were for. Also, the extra 40mm base you get in the box, whilst nice, doesn't appear to have any function. Apparently you make Ripper bases with them.
I think I may have glanced over the rules for Ripper Swarms once or twice but hardly gave them a second thought because the other army I play is Necrons. Doing a direct comparison between Ripper bases and Scarab bases can lead to codex envy. At first glance they are very similar but Scarabs have double the Leadership value and an armour save which is twice as good. Much more significant is the fact that Scarabs are 'Beasts' and have the 'Entropic Strike' rule. This means that they eat Land Raiders. Add some Canoptek Spyders into the mix and they can be fearsome.
Conversely, Ripper Swarms are plodding infantry with the 'Instinctive Behaviour (Feed)' rule and cannibalistic hunger. This means that, left to their own devices, they eat themselves. Inconceivable! It doesn't even fit with the fluff imho, since they are seeded across the prey world in their billions and are unlikely to be under synapse control most of the time. Their specific purpose is to consume the target planet, not each other. Its inefficient is what it is. They don't even have Move Through Cover which most Nid troops get. FFS, they are little, squishy, wormy things and they can't move through cover? And no, the 'Swarms' rule isn't as good. They can die by crawling up to the surface from underground and template/blast weapons make them pop like maggots in a fire. Idiots!
This has led me to remark, on several occasions, that Ripper Swarms are a pile of rancid dog turds and I would never even consider building the model, much less use them in a game. However, the changes in 7th Edition have caused me to eat my words (literary cannibalistic hunger?). First off, Rippers are troops, not fast attack. They are objective secured units and can not only claim objectives but can actively stop your enemy from claiming them. Secondly, you can make them deep striking troops allowing you to score very easy points in Maelstrom missions and get line breaker to boot. Thirdly, they are such a piss-poor, low-level threat that your opponent generally can't be bothered to kill them. If he wants to shoot them off the objectives then take note that they are fearless, have the smallest profile of any Tyranid model and can hide from line of sight very effectively.
I actually used them for the first time ever in a game the other night. They were my most valued unit, in mission terms. I got the 'Domination' tactical objective which means that you have to score every objective on the board at once. Normally I would discard it for the ridiculous nonsense that it is but this time I deep struck my 2 Ripper units onto remote objective markers and actually achieved it! The fact that I had to suicide my Gargoyles and a unit of Termagants to get all 6 is neither here nor there. They went on to score 6 out of my 10 points in the game by just sitting there and occasionally nibbling each other out of sheer boredom. Cannibalistic hunger is laughable on ripper bases. You can even give them wings if you want to. Pigs might fly indeed.