I think Sudden Empower is only 1/day, which considerably drops its usefulness in my opinion.
Rapid Spell I like, though not enough to bump my summons down a level. But as a rod, or combined with a metamagic reducer (metamagic spell focus (conjuration) is what I tend to end up using), it's a very good feat. You can offset this with a ring of the beast from CC, however (preparing SNA III with Rapid spell in a 4th level slot, and the ring of the beast makes it SNA IV with rapid spell).
The problem as I see it is that summoning druids are already feat-hungry. You want/need augment summoning (and probably spell focus (conjuration) to get that), greenbound summoning (if allowed), ashbound summoning (if EBCS is allowed), natural spell, rashemi elemental summoning (if Faerun material is allowed), the summon elemental reserve feat, rapid spell, a metamagic reducer for rapid spell, AND any other good feats like companion spellbound, quicken spell, or dragon wildshape.
Unless you're human and have a pair of flaws, it's tough to get the feats you want in a timely order as a summoner druid, which makes picking up Rapid Spell something I'd wait a bit on. Just my 2 cents.