Two words: Face. Splat.
At 2 Strength for 5, Saisentan is an expensive rez, even among other Sentries. It's closest companion is Neural Katana which is 1 more strength, for 1 less to rez, though 2 fewer subroutines. And that difference in subroutines makes all the difference. If Sensei-tan didn't have it's Encounter power, then it's a question of Strength cost vs Break cost
Some notable examples include:
Mimic breaks Katana for 1, vs 3 for Sai-sentence.
GS Shrike M2 breaks both for 4.
Ika breaks Katana for 3 vs 2 for Caeser-Ten; assuming already hosting.
The summary is that the credit tax to the runner is roughly the same, favoring Sentai-san only where breaking is more taxing than boosting, typically with static breakers; though I'll leave an in-depth comparison as an exercise for the reader...
But there's a wrinkle in all this I mentioned earlier. Thaisentan isn't actually JUST 3 Net damage. If allowed to fully fire, this card will land up to 6 Net damage. SIX. Realistically, it'll probably only land 4. Only.
Which brings me around to my initial two words. If you can pay for the rez cost, this cards puts an unprepared runner to well within flat-line range. In-Faction, this is a dangerous little Sentry, with a premium price to pay for it's premium package, and at three influence, it's probably not going to see use outside Jinteki, though a few of you will probably prove me wrong.
All this being said, is it worth the slot? From me, it gets a firm Maybe. It's ability and subroutines make it physically painful to move through, and while it's always going to be cheaper for the Runner to crack than for the Corp to rez, it makes an excellent face-blender in the early game. If you can afford the cost early enough to make it worth it, much like another high-rez-to-low-strength Sentry... Well, you're probably not looking to win by scoring.