I think there are a lot of us with that sentement!
The issue with T38 needs to be divided into two camps. Many provider tell you that you can send T38, but once they pass it up to their upstream providers which are usually LCRed to some level, it can fail. Even when they screen and interoperate with a provider, things change and it can fail. On SIPStation we will support T38 'for free' if you want to send it up to the upstream providers and take that risk. We also provide a metered T38 option (by sending us a specific prefix) for North American phone numbers and in that scenario we 'guarantee' end to end T38. We do this by fully controlling your T38 transmission to our gateway where we deliver it direclty to our PRI's. This puts us in full control of the problematic path and not at the mercy of the providers. It's expensive (for us) but allows us to provide an optional and reasonably priced option for dependnable outgoing FAXing if our customers want to choose.
For incoming, T38 poses another problem and the choice of carrier, their level of customer service, and their technical strength and relationship with their vendors becomes much more important. There are carriers who will provide T38 for their inbound DIDs. We have that in SIPStation. However, no matter what they do to 'guarantee' T38, the problems are not always in their control and if FAX is critical for your business, choosing a carrier with a deep understanding and the ability to work well with upstream providers is critical. A vendor who 'guarantees' T38 is still at the mercy of the originating providers who hand them off calls. As such, this can lead to failed T38's if the origianting carrier rejects requests (488) to convert the intial call to T38. Even with initial interoperability testing, things can change. When that happens, your carrier may be in a position to still guarantee you T38 but that means when they get a rejection from the originating call, they accept it as ulaw (or alaw) and then they convet it to T38 from their SBC to you. However, that can lead to partially or fully failed faxes. It's usually better then sending it all the way to you as ulaw/alaw since it's a shorter path, but it still has issues. In a scenario like that, you want a carrier who will investigate beyond a simple reply of "we sent you T38, faxing has issues, deal with it." You want a carrier who is capable of investigating the full path of the call to try and determine where issues are existing so they can be addressed back at the source.
Given all of that, a provider who fully understands all of this, and is able to work with upstream providers, often 2 or more levels away, is going to save you days of grief when running into issues. It doesn't guaratee that there will never be problems, as that's impossible if you don't have end to end control which no one does, but it gives you an experienced team to help you get back up and working when the problems surface as they will. It's also why many of the FAX services available sometimes seem a bit expensive, as there is significant resources, both in backend technologies as well as manpower to keep everything functioning so that you can run your business.
If you can get away with a cheap analog line, and have no need for "PRI" type capabilities (multiple DIDs, multiple simultaenous non-blocking calls, etc.) then that is often an excllent solution and likely the recommended one. However, there are often other important factors as well as a desire to have an eFax component of the service in conjunction with a real fax machine attached.
As has been previously "plugged" by James above, we (Sangoma) have some specialized FAX techology through the FaxxBochs technology aquistion made earlier this year. Keep your eye on SIPStation though as very shortly we'll be introducing that technology within the SIPStation product and we'll be providing pricing options that address the 'lower capacity' side of the spectrum to address the need of reliable FAX for the occasional faxing needs.