I used this for the first time today, and my God... the amount of damage it caused was just unreal! The ships which were destroyed and the ones that survived seemed to be a bit random though... about 25% of the ships in the blast radius survived, including some relatively light frigates. So, I'm wondering if anyone knows what determines survival against 'last resort'? Is there a maximum number of ships which can be affected?
I was playing a 7v1 against the AI and was a little over 2hrs into the game, turtled into my own little empire behind a single choke-point planet (which is the only way to win against 7 hard AIs really). I had just bumped my fleet up to a point where I thought I could break out (16 caps, 50 heavy cruisers, 30 light carriers). The enemy had 3 planets surrounding my choke world, and each of the 7 had their main fleet in one of these three planets. I picked the most suitable world to attack (had 2 ~1000 FS fleets garrisoned), using virtually all my forces, and within a minute of me jumping out the AI sent in a fleet. This was soon joined by another 4 fleets who quickly worked their way through the 30 gauss turrets to get to the fully upgraded starbase.
When this happened I jumped my cap ships back in (for the experience ) and when the starbase health dropped to 3000 (took about 1 minute I guess) I hit the 'last resort' button. In a flash I anhiliated 75% of the 5 enemy fleets, of around 1000 - 1200 fleet supply each. Before the blast my cap ships were mostly level 1, and immediately afterwards the majority of them were at level 5 or 6 (!). I brought in the rest of my forces and just managed to clear out the remaining forces, as well as the further 2 fleets which jumped in from the planet I tried to break-out to. After this I had 16 cap ships between level 6 and 8 (and my original cap ship at level 10).
Anyway, I have a newfound respect for the TEC starbase! I was thinking it was a bit lacking compared to the vasari (movement) and the Advent (meteor), but what else in this game can take out 5000 fleet supply all by itself?! I know that against a human player you will never get close to that, but this could be the 'easy' way to beat massed AI opponents. Garrison a border world, then jump out (if the enemy has a large force nearby they usually seem to attack within a few minutes once your fleet leaves). Draw them in close... and then BOOM In my game the enemies never did really recover. By the time they built their fleet up again I had taken the territory of my closest rival, and now had 3 separate choke points which never got attacked by more than 2 full fleets at once (defendable with mines, SB+gauss, and ~800FS of defence force). I also got 4 starbases in the star, which the vasari enemies seemed to just love throwing their forces against.
Anyway, the moral of the story is: last resort rules!