I have a Senco - had it over 10 years. It lives in my carpenter box (steel 28" long carry type" and only comes out when the application suits it... very infrequently.
It has it's place - used it to drive some 16 penny nails into deck framing that was obstructed too much to swing a hammer last summer. I don't think the heads on 6" spikes would fit in the driving end - it's recessed (as every other I have seen). Pressure on the center from the nail head opens the valve allowing the hammer to, well, hammer the nail and when the nail is flush the outer rim allows pressure to come off the inner from the nail head... the rim also keeps the nail head on the driver. I'd be using a waffle head framing hammer, 24-32oz for those spikes unless you're really good with a hammer in which case the suggested 2-3lb drilling/engineer hammer would be my suggestion... FWIW I would probably use the waffle framer myself - plenty of leverage and force and the waffle gives grip on the nail head giving more control. I've done a fair bit of construction and remodeling - my 24oz waffle framer and 16oz finnish (regular claw) live in the aforementioned tool box. My 2, 3, and 4lb drilling and engineers hammers live in the mechanics box... gives you an idea of the usefulness of them on average construction work.