I'm just curious, what's the worst surface you can take your cross bike over for extended periods. Not when racing but just out riding. Pics would be helpful if you have them but if not then just a description.

I'm just wondering if a cross bike is up to what I want or if I should just get an MTB.

What do you mean extended time period? I basically ride my cross bike on the same trails as my mountain bike, but with less jumping, a little more walking, and much more careful attention paid to my lines. The trails I avoid are not so much because they are too technical but because the climbing is too steep for me to accomplish with a 42 x 25 low gear.

Obviously I wouldn't take my cross bike to a ski resort in the summer...

I think it can handle some pretty nasty stuff, but I don't think it's fun. I'd rather ride a MTB over technical stuff.

What I ended up doing is I have a cyclocross (Specialized Tricross Comp) that I use for road riding, commuting, hard packed trails/rail beds and of course cyclocross. I have a 29er that I use for off road stuff, but even that I don't do highly technical stuff. Mainly just XC type stuff.

Any thing that won't pinch flat those skinny tires. Running 80psi to avoid pinch flats kind of takes some of the fun out of trail rides.

i have ridden my 'cross bike on some gnarly stuff, as for several months i wanted to go MTB'ing but didn't have an actual MTB. you can ride virtually anything you can on an MTB, but crank clearance is not as good for log jumping.

also, i didn't find it very fun. my hands would get beat up after 2 hrs or so of having a kung-fu death grip on the bars. no shock absorption. traction is also rather limited, as you have to run high PSI to avoid pinch-flats on rocks/roots.

I take mine almost anywhere you can take a mountain bike, and have a blast in the process

I'm a newb to XC bikes and I just returned from New Mexico where I put my Soma Double Cross through some testing. I've ridden the bike for about 3 months solely as a morning trainer, this was the first time off road. I first tested the bike on a State Hiway that has a nine mile section of dirt, and is actually a "primitive" road. That means that there is no actual built-up roadbed, it's just a road over the ground. I think it is occasionally graded, but it hadn't been in awhile judging by the condition. The bike performed flawlessly, it was a blast as I gained some confidence and skills dodging rocks and ruts.
On the next outing, I did a 90 mile loop with 20 miles of primitive road. Some of the road was packed sand, some loose, lotsa ruts, and many short rocky sections, also with ruts. Again, the bike was a blast and performed well.

On my 3rd outing, I rode up a Nat'l Forest road. This was actually less than primitive, with some really steep rocky sections, one of which I had to portage. The combination of football and basketball size rocks with my 34/25 gearing was just a little too much for me. I rode for about 8 miles up on this road and climbed from 8,500' to over 10,500', then I rode down. I also tried a little single track, and with my improved skills, I was able to ride that with little difficulty.

On my 4th outing, I decided to try a trail that is partially rated expert. The first part is a 700' climb in about a mile and a quarter, maybe a mile and a half. I had to walk/portage several sections, I just had the wrong gearing. The loop was about 8 to 10 miles with a total elevation gain of about 1500'. I'd estimate that I had to portage about 1/2 mile total. The last part of the trail about 1 mile) is old logging road (double track) and was pretty easy especially considering that I was going downhill or flat for most of that.

I rarely mtn bike and it has been several years since the last time. I learned a couple things on the CX bike. I definitely need to get a 12/27 or 12/28 cassette. That alone would open up some sections that I stalled out on. I am also thinking that I could use some of those auxiliary brake levers up on the flats. Riding downhill for long sections in the drops is hard on the shoulders and arms, and makes proper weight distribution a problem too. I was riding on Ritchey Aero wheels and Ritchey Speed Max 700/32 clinchers. A larger tire may have been to my advantage also. All in all, it was definitely fun and the bike was more versatile better than I expected. I'm looking forward to more off road adventures.

Zagnut

I've recently purchased a SS specialized tricross. I decided to "test it" on some mountain bike trails around here to see how tough the bike was. I beat it up pretty hard and never had a problem. My only complaint was the pain in my palms from riding w/ my hands up on the hoods over the rough terrain.

Horses for courses.

Sure, you can certainly pick your way through rocky, rooty singletrack on a cross bike, but it can be much more fun on a true fat tire bike. If that were the majority of riding you did, you'd be silly not to get a mountain bike.

On the other hand, if you are riding for extended periods on fire trails, gravel roads, and the like, then a mountain bike feels sluggish and overkill, while a cross bike is perfect.

Where are you located? The "technical" trails are very different between Minnesota and Connecticut, for instance.