VoltaicHD
I didn't have iMovie '09 so I had a look around at the additional options before laying out the cash: Toast ($$), VoltaicHD ($) and Handbrake (0).
Nutshell:
VoltaicHD won, and I recommend it despite its looks.
The strength of VoltaicHD (+ iMovie '09, '08 or iMovieHD) over
only iMovie '09 is that you can keep all those highly compressed .MTS files (= AVCHD including AVCHDlite) as-is on your hard drive any way you like and uncompress for editing in iMovie as needed. Like having negatives. iMovie '09 requires that you import from your camera (or that your "fake" the directory structure of your camera) and decompresses everything so movie files take up to 10X more disk space than the original MTS file. I'll likely pick up the next incarnation of iMovie ('11?), but for now I'm happy with '08 and VoltaicHD. It requires a workflow to manage my MTS files + iMovie projects, but I save heaps of space by deleting unused portions of video in iMovie with reckless abandon knowing everything is in the MTS files I've saved. Naturally, conversions take some time. I usually set-up all AVCHD clips I shot in a day for
batch conversion, go tend to some other business, and invariably they are done before I remember to check on them again.
Handbrake lost because despite the bazillion possible configurations, none produced as good decompressions as VoltaicHD's basic mpeg-4 setting. No fussing, no guessing. Oh, and batch conversions. I'll repeat that: batch conversion. Batch, batch, batchitty batch-batch. Batch.

. The developer of Handbrake (at the time I looked into all this, may have changed since) was dead-set against implementing batch conversions. He had spent a considerable time in the user forums abusing people who requested this simple feature. He couldn't wrap his head around the idea that there was a growing army of Mums and Dads each with hundreds of 30sec AVCHD clips sitting on SD cards in need of conversion. They didn't want to spend as much time clicking through settings for converting each clip individually as they did filming the damn things. However, you could always suffer through the command line interface and wrap a batch routine around your favorite settings if you are into that sort of thing. I'd be interested in hearing such success stories ('cause I am into that sort of thing). But hey, Handbrake is free and is useful in other ways, so there's that.
Toast? It was just awful.