Since this thread seems to be on the pessimistic side of things, here's my viewpoint skewed to that demographic
The biggest problems I see with TDM and Digital systems (generally):
1) It can take days, weeks, months, to add lines to a non-VoIP system, depending on the PSTN demarc type (T1, POTS, etc).
2) New features (not fixes) are usually forklift upgrades
3) Everything is tied to number of ports, not bandwidth
4) Cost for Telco is usually more expensive then Data (think multiple offices)
5) A digital system can only extend to it's local devices, or otherwise limited to the reach of it's interconnects
6) A MAC order must be done to move an extension
7) Often seems like it requires a wall of add-on products to get it to do what the customer really wants (IVR, MoH, etc)
8) Doesn't necessarily play well with other systems
And here is what I don't like about VoIP, and it's variants:
1) Economy of Scale non-existent in small business
2) Requires proper internal network (not necessarily more maintenance, or split network)
3) More complex, making diagnosing more like engineering then trouble shooting
4) Requires quality, broadband internet
5) Router/NAT issues tend to pop-up
6) Not all features work the same (SLA for example)
7) Artificial Limits I.E. Licensing and Subscriptions
8) Potential ancillary costs not anticipated at quote
9) Some SIP "Carriers" can be a real joke!!!
Wish I could comment on Hybrids, but I have never dealt with one. I guess it would be correct to say it's the best and worst of both worlds, like a monkey in the middle. I guess if I was out selling phone to the world I would probably try to pitch a Hybrid most of the time, that way they can have digital simplicity where it makes sense, and the fancy VoIP stuff when they want. Seems like the best option from where I stand.
There are some very strong use case scenarios for VoIP, but they almost always apply to enterprise use or at best medium sized business'. Most small business, which is the bulk of business', will never reach those kinds of levels. Nowhere can this be better seen then in the uncanny resemblance that SS7 and SIP have to each other, with the exception of SS7 circuit-switching the audio and SIP packet-switching it.
About the only major selling point I see for VoIP in a small business is a case where you don't have (or don't want) a brick-and-mortar office. Your average "Executive Office Hotel" comes to mind here. We got a lot of them around here that lawyers buy into. 95% of their work is done at home in their garage, but when they need to meet a client they schedule time to occupy a suite. They have all recently gotten into the virtual-office deal where a receptionist sits at the lobby and answers your line for you. She says whatever greeting you want and can check to see if your phone is DND or not. The rest of the time you get a soft-phone at home or a hard-phone on the desk, and the callers get a simple voicemail drop box after hours. It's all about perception I guess.