I'm glad that there is a clear feeling that 158's are in fact generally good trains, and I would certainly agree. The 158 is a solid and dependable unit, and represents the best of the late BR stock. I would certainly rather travel on a 158 than sit in the nasty low-rent interior of a Turbo or Networker, or a 323 up north. A well looked after, nicely upholstered and liveried 158 set will offer a travelling environment just as good as any new DMU. As somebody has already pointed out, you need to travel on a SWT 158 before judging, don't assume they are all as shabby as certain other TOCs' examples. The SWT refurbishment and ongoing excellent maintenance demonstrate the dramatic difference between operators who are willing to care for their trains, and those who view them simply as a chunk of metal with a job to do.
As for the aircon issue, it was initially designed to run on CFC-based chemicals, which are now illegal. However, the system can and will still work effectively, but again, it needs to be looked after. Too many TOC's just can be arsed to fix it when it needs attention. The toilets aren't wonderful, and they do sometimes lose the fight with the Richard III's, but this is easily resolved by fitting new equipment at refurb, something which is not unreasonable when the existing stuff is 20 years old! Again we come back to SWT, who fitted chemical retention equipment on their 158's several years back now. The perceived problem with sloww boarding on busy services is unavoidable really, and I certainly prefer a door at each end of the coach on a long run, rather than sitting on a glorified suburban train.