Why are some so keen in replacing OnePage?
The answer is easy.
It is an attempt to reduce checkout funnel exit and increase conversion.
But how you've come to the conclusion that the checkout process itself is the issue, and not your shipping methods/pricing, required fields etc. is the important matter.
Selecting a checkout isn't your decision - your customer decides, well, they don't know that - but indirectly they do.
If you were to measure your checkout goal funnel and review losses - you could start to build a picture of where and what was causing problems.
There isn't anything wrong with the standard Magento One Page Checkout (the "Accordian") - it functions as it should. But its appropriateness to each store is what is under question.
- Does your target market have limited patience?
- Does the checkout need to appear to be "simpler" or "shorter" to keep people interested?
- Are you perhaps trying to cater for those familiar with Ebay or Amazon to mimic a similar appearance?
- Are you trying to reduce confusion/increase usability throughout the process?
My personal opinion, from experience, is that the 3rd party checkouts are not worth the extra hassle they bring, mainly in extending their functionality.
Perhaps you have either probably picked them for the wrong reason - or not sufficiently A|B tested the checkout process with alternative methods to come to a proper solution. In a given situation, for a given store, an alternative variation of the OPC could have dramatic increases on conversion.
But you don't need to install a 3rd party extension to alter the appearance of the checkout process. Simple CSS edits can make a substantial difference to the flow and appearance of the checkout without compromising upgrade-ability.
What might be appropriate for your products and customers, certainly isn't applicable to anyone else