I've owned an 05 ION since new. The Cobalt was based on the Delta platform introduced with the ION in 03. The Cobalt had 800 or so improvements that it shared with the ION in 05.
While it may be hard to fathom today, the Delta platform designers used the VW Jetta as their engineering objective.
Sway bar bushings have been a problem. Run the VIN for recalls as there is one for replacement of the electronic steering motor, and of course the ignition switch.
The ION has been inexpensive to run:battery lasted almost nine years [it's in the trunk], tires and oil changes, regular servicing. Still looks like a late model. Always beat the EPA #s as published in 05. The 10% hit EPA did on all it's ratings are even more off the mark.
Sonic 1.8 has a timing belt. Needs more frequent servicing than the 2.2/2.4. The 1.4 turbo uses a chain.
2.2 Engine is fine and the GM 4 speed auto is common and has been perfectly reliable. I keep both serviced using a severe duty schedule and Mobil 1 in the crankcase.
People bag on the Cobalt now, but the automotive press of the period thought GM & Chevrolet did a fairly decent job with their "premium" small car. Especially given the Cavalier that preceded it.
The 95 SL1 5M I bought with 116,000 miles was still so solid, that when my 99 Cavalier was totalled [also purchased new], a Saturn ION made the list. The Cobalts were overpriced and overloaded and thinly stocked at the Chevrolet dealership, and since I didn't want a truck or SUV, which were the bulk of the inventory there, I went to the Saturn dealer and found exactly what I wanted.
I had considered the Neon briefly, but by that time crash stats, depreciation and reputation put it at the bottom of the barrel in the segment and I've been a longtime Mopar fan. Too many negatives for me to overcome, even though the Cavalier swam in that same low end.
I will say that the manual in the ION I road tested sucked compared to the manual in the S Series. Had it been as instinctive to use as the SL1, I'd have purchased the manual over the automatic.