I rode one, but...
I rode a tensor's, and thought it was great fun (let's see, purple wurms, tremorsense.... let's float along in midair on a tensor's!!) But that doesn't change the fact I think riding it is overpowered. I don't have strong reasons, but several weak ones taken together:
1) Compare to Rabbitfoot, a 1st level druid/ranger spell in Scarred Lands. Rabbitfoot gives +2/caster level, max of +10, move silently bonus. A Druid casting it at 1st level gets a +2 bonus.
Tensor's disk allows the user to float along completely silently for hours. A wizard casting it at first level gets an unbeatable move silent bonus. It should be pointed out that this is not a perfect substitution: it does not allow the user to manipulate objects any more silently.
2) Comparison to levitation/Waterwalk: Already discussed.
3) Nullify Tremorsense: ridden, the disk completely counters tremorsense. not bad...
4) Higher ground: If 3' up in the air doesn't give you +1 melee for being on higher ground, I don't know what does.
5) Traps: most traps will not be triggered by someone floating along on a tensor's... they are pressure sensitive or have tripwires within 6" of the floor. Most things a Halfling can walk into a disk can float over.
6) Flexibility: Combine all the advantages just given with the fact that it can also be used to nullify load requirements for your entire party/save you the cost of a packhorse.
Not to mention the other creative uses of having an instant stepladder around (+3 foot vertical jump, for example).
Taken all together, these make riding the disk seem pretty potent, and importantly, pretty flexible. Flexible enough that it's worth it to memorize a tensor's every day, almost irregardless of what sort of activity is being engaged in or what level the party is at. And that is why I think it's overpowered.
That having been said, without an official ruling it's up to the individual DM to determine if an overpowered ability may be used.