I've been playing Tekken for a very long time. When Tekken 5: Dark Resurrection was released on PSP it was one of the most fully-featured Tekken games of all time. Tekken 6 PSP is unfortunately a step down from that.
First off the graphics are amazing. It seems that they imported the Tekken 5 character models into Tekken 6 with some more lighting features, this is fine as the Tekken 5 character models on PSP were amazing already. The model of the new characters are on par with the rest of the cast. What truly takes the graphics up a notch are the stages, they feature the same animations and functions as their arcade/console counterparts, obviously downgraded but still an amazing sight to behold.
As for gameplay the actual fighting is 100% the same as the arcade/console game. The only thing different are the features. They seemed to have skimped out on a few things. At the end of every round in Tekken 5 PSP there was a slo-mo replay that played, this is missing on Tekken 6 PSP even though the arcade/console game still retains this feature. Not a big deal but it's something that Tekken has always had and something I enjoyed looking at when there was an awesome looking finish.
As for sound, oh man this is a definite dissapointment. The sounds themselves are the same as the arcade/console game but the sound quality is TERRIBLE. I compared this to the Tekken 5 PSP game side by side and the Kbit or whatever its called is drastically noticable in difference. You get that slight sound of static in almost all the sound effects. However if you haven't played Tekken 5 PSP chances are you won't know the difference.
Finally the customization, there are a few more options than Tekken 5 PSP but this was expected. However the robust customization seen in the arcade/console version is reduced drastically to some pre-set costume pieces with some colour options. Not much you can do there.
All modes are there as you expected. No more Tekken Bowling as featured in Tekken 5 PSP :( but things like Arcade (Same levels as arcade machines.), Story Mode (Impossible to understand why you fight the people you do, is explained if you play the console version. Contains prologue and an end cinematic for each character.), Gold Rush, Survival, etc.
Overall Tekken 6 is still a solid PSP port. It's a step down from Tekken 5 PSP but that's because Tekken 5 PSP set the bar so high. Serious Tekken players will use the Tekken 6 on PSP as a training/testing tool on-the-go until they can get on an arcade or console. Not-so-hardcore players will find a lot on offer here in terms of gameplay. Really when you play story you just want the cinematics anyways and in that respect this delivers very well. ;P