If you're into strategy RPGs, the PSP is your best friend in the world right now. Currently that blasted Jeanne D'Arc is sucking away every ounce of my free time, and both Disgaea AoD and Final Fantasy Tactics WotL are incredibly good renditions of the PS1 classics, especially if you've never played either before (I'm a first time newbie to Disgaea, and thus far I'm finding it to be one of the most addictive [and hilarious] titles I've played in ages). I finally finished playing Puzzle Quest after clocking in like 40 hours, but I'm already feeling the urge to return to its bizarre Bejeweled-meets-Final-Fantasy world of wonder.
Beyond that genre, there are several other more recent PSP titles that I've found quite enjoyable. Castlevania: Dracula X Chronicles is excellent, especially if you're as big of a classic Castlevania fan as I am. Silent Hill: Origins is every bit as good as the original (but not a huge step forward, I must admit). Syphon Filter: Logan's Shadow is even better than its predecessor. Star Wars Battlefront: Renegade Squadron is a heckuva lot better than some reviewers would have you believe. Brave Story is pretty good if you're in the mood for the most generic RPG you've ever played in your life, after Final Fantasy 3 for DS of course, albeit with vastly superior graphics. Manhunt 2 is decent, but not really worth the full retail price, if you ask me. Wipeout Pulse is probably the best entry I've yet to play in the entire series, but if you're not a fan already it certainly isn't going to convert you.
As for the DS's more recent affair, I must say that Revenant Wings is among the best titles I've ever played on the system, and certainly much better than most reviewers are saying. Orcs and Elves is fairly addicting if you can overlook the hideous graphics, but it's nothing incredible (pretty much the same as the cell phone version, which isn't a bad thing per se). Phoenix Wright: Trials and Tribulations easily wins my "best in series" label. Dementium: The Ward is another title that's much better than reviewers would have you believe. Dragon Quest: Jokers seems pretty darn fun, but I've only played about an hour or two of it so I can't really speak for the entire game.
And, of course, if you haven't picked up Castlevania: Portrait of Ruin as of yet, then there shouldn't be anything holding you back; it's not quite as amazing as Dawn of Sorrow, but it's pretty darn close if you ask me.