What is serious?
Darn if I know. There are several definitions I could think of:
1. Am I/Why and I serious? I have no idea.
a) Serious mind set... plans trips carefully and brings what is needed to come back healthy. Doesn't turn tail when it looks like bad weather. This is oppose to the carefree backpacker who calls for a rescue because they are feeling a bit cold or tired. By this, serious.
b) Serious devotion... measured by the number of days you spend backpacking. I suppose it depends of the cutover. There are people here who report type spend 300 days in the back country. I am certainly not one of these folks. I have a regular job and a family. In good years, I was about to get in maybe 40 nights. Is that serious? To someone who does 1-2 weekends a year. Yup. To someone who spends 300 days? Nope.
c) Serious trips... measure by number of days?. What's the break point? A week, 2 weeks, a month, a non stop thru-hike? Seems like a hard thing to settle. I have taken multi-week trips (with and without resupply) in the past but not done a thru hike. I haven't gone unsupported more than 10 days. Serious? Depends on the break point, though I think this would likely quality as serious. On the other hand I haven't taken trip that lasted more than 6 days in the last few years due to family needs. So does this mean I used to be serious and am now not?
d) Serious because you carry a serious backpack or more than X weight. By this definition, I likely haven't been serious for at least 10 years, likely 20 when I stopped carrying 60+lb loads.
Bottom line, there are all sorts of dimensions one could use for "serious". I am not sure if it's very useful to think about.
2. What pack do you use for serious backpacking?
Depends on the def. of serious. Right now I can happily use my GG Gorilla for up to around 10 days unless dealing with heavy snow. If I HAD to carry more, then I would switch to my loaner pack which is the original Osprey Aether 60, though I am sure there are packs that are lighter than 3.5lb and can carry 40lbs comfortably. But it would have to be an insanely great trip to convince me it was worth carrying so much stuff that I had to break out the old pack. Beyond that... see next question
3. Do you still use something like this, or would you be in the market for one again?
No. I am not as young as I used to be, and my body carries some damage. I no longer climb or hunt, and I don't carry huge view cameras. I can't imagine anything that would get me to carry more than 40lbs for any length of time other than helping someone else. In the last 5 years, the only time I carried more than 40lbs was when I was carrying my pack as a front pack, and a heavy-weighters pack on my back for a couple of miles because he was carrying his daughter who wasn't up to walking any more on her own. [Hmm... maybe if his daughter had been wearing trail runners and carrying a 12lb pack like my daughter was, rather than the new boots and a much heavier pack, we won't have had to do this. Hmm... not carrying a big pack myself is why I was about to take his. Yep, no monster pack for me] Reminded me why I carry a light pack now. I would rather not carry more than 25lb but would for the first few days of a trip as I eat the food down. So maybe I am not serious. If I was serious I would be willing to carry nearly any amount of weight to do one of those "must go" trips.