Alright, I've gone through almost all the single-player content (Operations, Time Trials, Hostiles). Haven't played multi-player yet, but this is what I think so far.
The FPS engine is solid and the controls work well, including the touch controls. Feels very much like a portable COD game. But with little content, boring level design, and poor enemy AI. There's 10 Operations missions (~3-8 minutes long), 5 Time Trial missions (~0.5-1 minute long), and 6 Hostiles missions (as long as you can last). Of course there's replay value in getting high scores, but I'm not really feeling driven to replay the missions. Mainly 'cause they're not so great.
The levels are linear, as expected, but they're also cramped, lacking variety, and devoid of any creativity. The enemies don't do much except run into position and stay there, or sometimes they run right toward you. There's also a few occasions when the enemies spawn awkwardly close by, which is weird, but whatever. If you just want some mindless fun, Declassified is satisfying enough. It's just kind of a shallow, paltry single-player experience, much more so than other COD games.
I imagine the multi-player experience in Declassified is somewhat deeper. Not nearly as feature-rich as the console versions, but it's the best portable option, which is unfortunate.