I’ve bought some of the Sega #D classics for the 3DS.  Here are some quick thoughts/recommendations:

• Space Harrier:  If you’re into “retro” arcade games or just fast-paced arcade games in general, this is well worth your $6.  Best version of Space Harrier I’ve seen outside the arcades.
• Super Hang On:  Another retro arcade game, and another worthy purchase.  Gyro controls are surprisingly fun with this title.
• Galaxy Force 2:  Fun, but this game will chew you up and spit you out.  Expect to die.  Excellent use of 3D – probably the best 3D in all the classics released so far.
• Sonic:  it’s Sonic 1 from the Genesis with minor 3D enhancements and the spin dash.  Get this if you want digitally-available Sonic 1 on your 3DS instead of on your phone.
• Ecco:  Ecco’s definitely an acquired taste.  This is the Genesis version with minor 3D enhancements.  Get if you love Ecco already, otherwise try one of the other classics.
• Streets of Rage:  Pretty fun (though very basic) beat-em-up.  3D’s actually quite useful in this game in showing where you are relative to enemies.  A one-hit-kill mode has been added if you feel like just blowing through all the stages.  This one’s great fun, but we really need Streets of Rage 2 … now!