In my opinion yes, the signature places are a bit better than most single credit TS with some exceptions.
In my opinion, Cali Grill and CRT are two places that very much are NOT worth two credits.
Most of the single credit buffets have really gone downhill in the last few years.
DDP decreased the quality, and now the recession. Disney seems to have really cut back onthe food quality. Which is such a shame.
There are a few exceptions. Tutto Italia was excellent on our most recent visit. It was, in my opinion, better than Cali Grill. The one exception at Cali is that some of the sushi is excellent. The 'sushi deluxe' there was nothing special, but some of their specialty rolls are really great. The specialty rolls don't make a great fit with DDP though. The desserts at Cali are also outstanding. The entrees though just were not very good at all. Service can be mixed. The staff tends to be too high and mighty, IMO. The check-in staff in particular - compared to other Disney staff - just are not friendly or helpful.
They just act like they can't be bothered most of the time to answer even simple questions, much less give any info beyond a direct question. (Like when my dad inquired at one restaurant at GF, they suggested that Citricos might have a table available, and offered to check. At Cali, when they are booked solid, they can't be bothered to direct guests to try at the Wave, much less make the effort to actually look up tables there.) It's just such a contrast with almost everywhere else at WDW. Eventhe check-in staff at the value resorts - WDW's least expensive resorts - go out of their way to be nice and helpful most of the time.
We weren't that hot on Brown Derby, but it wasn't bad in any way. It just wasn't our taste in food style.
Our most recent meal at Narcoossee's was excellent! I also like Flying Fish. We recently enjoyed dining at Citricos. Only one person thought their food was just okay; everyone else enjoyed it very much.
There are some I haven't tried: Artist's Point, Jiko.
I haven't dined there in the last year, but one reason we haven't dined at Jiko is that we like Boma so much!
I will give one caveat, as other posters have stated - meals at siggy places can easily take two hours. That can be a bit much for young kids, especially as it gets later and kids tend to get tired. You will tend to see more families dining at such places on the earlier side.