He has 6 attacks on the charge, which should net him one extra attack. When not charging (he is slow moving) It's around 0,6 extra attacks. He will usually get one rending hit every other turn. VS marines i he killing around 1.3 on avg each round, and vs termies less than one. VS MC he is decent and pretty good vs high armor, but if you are hitting rear armor 10 you are gonna destroy it regardless of seth. Slow moving as well. Just a single str 8 hit will instakill him. It's very hard to utilize rampage as he is reliant on protection from a large unit. If you are playing vs Ork's he looks solid, but vs most armies he has so many weak spots, even if he is fairly priced.
You can get a Captain with 2+ 3++ T5 (bike) and a relic sword for only a few more points.