I'm one of the "lucky ones". I had a 2003 YZ250F, rode it for three years, valves were still within spec when I sold it. I threw a new piston and timing chain in before I sold it. The guy I sold it to is nowhere near as thorough on maintenance as I am, and she is still blasting strong (it's 8/2009 now; six years of hard use, one piston change) .
Same thing with my 2006 YZ250F; I put in a 262 kit (2mm overbore), and then after a year or two replaced the piston (just to see what a lighter weight, shorter one would do compared to the taller skirt one). Valves still perfectly in spec, no reason to do anything with them.
Doing the top end really isn't very hard. I am a terrible mechanic. I learned by doing top ends on my two-strokes. If you can do that, then the four-stroke is not much harder, just a few extra parts/things to do. I think people get intimidated, but seriously, it's a single cylinder, not that much going on in there when you get into it.
Is a catostrophic engine failure more costly? Yes, I think it would be. I wouldn't know.
So for me, compared to my two strokes, the cost of doing the maintenance did increase, but only minorly (cam chain and valve shims are my new costs; the piston and rings are a constant for both). I'm not spending money on premix oil now, but I do find that I change my oil in my four-stroke more often than I did in my two-strokes. I consider that a wash then.
I prefer not having to mix gas, so I can stop anywhere without a can. I can just head out, every gas station has what I need. If I'm trail riding for a long time, and get low on gas, I can just pull up to a pump if we stumble upon a station.
I really like the smoother power of the 250 four-stroke. I have had a 250 two-stroke and a 125. These bikes were fun, don't get me wrong. The 125 was a blast to ride. The 250 two-stroke got me into the sport. These were very fun bikes.
When I got on a 250F, my skills got better faster. Why? Because I was no longer afraid of jumping the bike. It became very predictable. It jumps the same way every time, if I do the same things. On my 125, sometimes I would be nose-high, sometimes I would do an endo, it was way harder to jump evenly, because when the power hit, it was sudden and fast. Yes I know, if I could have figured it out, I would have been a better rider today. I rode two-strokes for about seven years, and I still sucked at jumping my 125 consistently. The 250 wasn't so bad, just not as predictable and easy to ride as the 250F.
I found that I was able to start paying attention to other things, because I wasn't so focused on the takeoff any more, that part got easier. So once I figured out that the bike was more predictable for me, I could start jumping farther and trying other obstacles. I had more confidence that the bike would do what I wanted it to.
Why take the "easy way out"? Why not just work harder? I'm 37, I'm not an awesome rider, I have no future in this sport; I ride for fun, and the 250F is fun for me. I do work out though so I can ride better, longer, without being as affected. I think riding is a healthy habit, it keeps me from getting too terribly fat.
Did I have great luck with my two strokes? Yes yes yes. I rode each of them for many years before I'd do a piston. As a sucky rider, I guess I just don't tax the motors as hard. My luck has been good with both kinds.
For now, I think it comes down to; "which is more fun for you?" Some people love the rip of the two-stoke. I do too, as long as we're not jumping it. No one is wrong here, that's the beautiful thing, we have choices. As long as we're riding, we're brothers and sisters of the dirt!
I think it's great to have a choice. I love to get on a two-stroke every once in a while, now that I've gotten a little better, I can ride a two-stroke a little better too. They are fun for real.
Now having said all that; you guys are totally right in saying that a two-stroke is superior technology. Look at the facts! More power in the same CCs, easy to figure that one out. Also totally correct in how easy changing a top end is, yes it's pretty simple. The design is more simple. If they made one that has the exact same hp/torque curve
as a 250F, didn't blow smoke (I ride indoors in the winter), and had oil injection so I don't have to remember to bring a can with me... now we're talking!
Anyway, in summary, the maintenance lie has been one to me; I think the costs have pretty much been a wash so far. If there were slight increases for me, it has not been huge. Has it been harder to change a piston? It takes more time, yes. Have I been able to do it, me, an untrained guy who just read the manual? You bet. I had an advantage though in owning two-strokes in the past, teaching me how to do the top end already. Maybe if I owned one of those 250Fs that is more prone to blowing up, this whole post would be a complete opposite. My Yamahas have been very reliable for me, rock solid.
(sorry for the book! way longer than I thought)