|How good are these in 8th?|
Each unit has it's good and bad points. I can tell you no matter which way you choose to go, you can't really have enough. I would consider 90 boyz and/or stormboyz a minimum for any army because of how fragile they are. Bikes though, can be fielded in addition to the others much easier.
Boyz are the basic troop that just has little downside. The best LD mitigation that isn't strait up fearless, and the option of either shootas or sluggas at no extra cost...which is darn cheap. They work GREAT with Da Jump to show up anywhere on the table, shoot and charge. If you are going to jump them I would recommend shootas over sluggas. The versatility to shoot rokkets at one unit, shootas at another (or 2) and then charge yet a different unit gives you a lot of versatility. With the great 'ere we go (and a command point to re-roll one die instead of both) you've got a GREAT chance at charging after jumping.
If you are going to just move across the board with a warboss for advance+charge and a Waggh! banner nob for 2+ to hit, then sluggas are the way to go. I would still get a wierdboy for Warpath and maybe even Ghazzy himself. Getting 6 attacks hitting on 2+ isn't hard to achieve at all. Adding a KFF mek and moving the whole army under the bubble will give you the survivability you need to get there.
Warbikes were by far the king of the previous codex. They took a huge hit with the increase of cost by 50% and loss of any form of exhaust cloud (they should always counts as being in cover, giving an effective 3+ armor save), and dropped from 15 to 12 per unit but gained a wound each. They are also much faster than even stormboyz with a 14" move and by far the shooty-ist at SIX shots a model - the result of removing twin linked from the game.
The cost of these guys really hurts in large numbers - a full unit with PK can creep near 350pts. That does not leave much room for other models if you go bike heavy. But the same 4+ save as before can help against small arms fire, and buffing them with a painboy on bike and KFF mek on bike will give them even more resiliency to get where they need to be. Their speed will guarantee they will be a threat that can't be ignored so I would say expect to lose them early as the opponent concentrates fire. Despite my love for warbikes and the benefits on paper, I can't see them forming the core of any army.
Lastly is Stormboyz. Sadly I don't have enough models to really playtest them, as 60 should be the minimum I would consider. They do require Zagstruck to go along with them, as he makes them immune to morale!! And at under 100pts it's an investment that's more than worth it. They have a lot of downsides IMO, being 50% more expensive than boyz and no harder to kill. They are fast, with a 12" move and able to advance+charge gives a 12+3d6" threat range (at a price, the potential to lose a lot of the unit) and without any more toughness so you'll lose a lot moving to charge range. But with Zagstruck you won't have to worry about morale and you can keep charging forward so your opponent has to kill them to a man!
The real question is which way to go? I think the ability to show up where and when you want, and then charge, is going to dominate the early meta of 8th. Orks may not have a lot of units that can do this, but the speed of some of the Ork units can make up for it.