The Harmony is not as flimsy as a Graco or Evenflo, but not as study as a Recaro.
We also have a Harmony folding travel booster, and I love the belt fit on both of my kids! It's not a flimsy booster...and while I understand the concern about boosters not seeming as "sturdy" as a harnessed car seat, we have to remember that the booster's primary purpose is to correctly position the adult seat belt on kids. I personally am uncomfortable with a really heavy booster, unless it can be latched in. Our Cybex booster is really heavy and sturdy...and we have used it when it wasn't latched in...but I much prefer it latched.
Anyhoo..the Harmony doesn't fall apart when you move it, it's light weight, and it does have additional safety features (deep head wings, air blown shell--forget what exactly that's called, but it's supposed to offer similar extra protection to EPS/EPP foam, a bottom shape that could make sliding under the lap belt less likely, etc).
Babies R Us carries the Evenflo Right Fit. I think it looks really nice. We have an Evenflo Amp high back that fits well in some cars and not well in others (lap belt rides too high in some cars, b/c of the seat belt geometry).
In all honesty, most boosters are not going to seem as "sturdy" as a harnessed car seat. They are designed differently. The most important thing is that the belt fits properly and your child sits right in it. If what works/fits is a $30 "flimsy" Evenflo Amp high back, then that's better for that kid and situation than a $120 Britax Parkway that the shoulder belt is too far off the shoulder of a skinny kid (like my son's--the PW actually does NOT position the shoulder belt correctly on them. I don't let them ride in my friend's. She bought an extra Graco to have on hand for them. (-