Well, let me take this in order.
Is there only a fat client ? Yes. There is no such thing as hooking a web browser to the server to do Notes design. The Administrator can do almost everything via browser since R5, but the designer is a long way off from that and I don't think Lotus has even put it on the radar.
Does Notes talk to Exchange ? Well, your mileage may vary on that, but generally Notes will talk to any SMTP source, so it should. I'm only a Notes developer, so I don't know all that much about Exchange (thankfully).
Finally, is Notes a good solution for a small company with remote users ? I believe the answer depends on whether or not you have the resources to support the mission you give to it.
Is Notes just for email ? Then you basically need an administrator. Is Notes to be part of your application base ? Then you need an administrator AND a developer.
If you do not have the resources, then Notes is not the right answer. Then again, if you install Exchange without anybody knowing how it works, I do not expect that you'd be able to use it for very long either.
Notes is great for keeping remote users in contact. Discussion forums and database replication allow for whole applications to continue their function even though the users cannot log in to the network all day. There is a great amount of freedom and power to be found in Notes, but only if you have competent people to extract it.
Any tool used by a company must be mastered for it to be useful to the company, whatever its size.
The only thing going for Notes is that you only need two proficiencies to make it work. And you do not need to upgrade and/or reinstall it when you change Active Directory or upgrade the Windows OS ;-).
Domino has a great many tools that allow a developer to not worry about how it works. An administrator only has to find the right setting to activate a Domino function - no need to buy additional third party software in most cases (except fax stuff - that can be a nuisance).
I think Notes is a great product - I have been working with it for almost a decade now. But I realize that it is different and slightly unforgiving for the clueless manager. In other words, if you have no support for the server, you will hardly go farther than email with Notes.
But as I said, I don't think you can install an Exchange server and forget about either.
Pascal.