I have never used one properly. Both my eldest kids have iPods and seemingly powerful phones, but we don't have a tablet in the house.
I remember the first time I saw one was just after the iPad was released in this country, I was sat in a café in Islington and a girl walked in, sat down at a table, and pulled one out of her bag and set to her work immediately like she had been using it all her life. She let me have a quick peek at it, and I thought it was an amazing bit of kit. I would never of dragged a laptop along with me to London for what I was there for that day, but would I have taken one of those if I had one.. quite possibly.
That said, the amount I travel far now is not often. It's not financially viable for me to buy one yet, it would only be a toy if I did, not a tool. My PC does for home stuff.
Small portable devices will increasingly become more popular I am sure. As for more powerful though? That's dependant on how you define power. If it's just raw processing power, functionality, and usability of day to day applications, if they don't match the average PC now I am sure they will before long. What I don't see in these devices currently though is redundancy. What do I mean by that? Well, what happens if something breaks? An issue with the power maybe, or the storage (all the apps and data you have on them)? This is where the larger home computers may come in to there own in the near future, having room for mirrored storage, dual power etc.
Deviating from the topic slightly, but just for fun, this beast doesn't exist any more.. we've parted it down. But, not so long ago this is something we created at work to mass wipe over a hundred disks at a time. It could though easily of been used as an always up system, having three power supplies to the core systems for double redundancy of PSU failure, two massive battery backups, and multi RAID arrays:
Small and portable I am sure will become increasingly more popular. But large non-movable units will still have there place for some time to come.