I've been using my smart phone(s) for SSH access for the last couple years. I run a small business and have very demanding customers (who pay a substantial amount for support), and therefore need quick response. The smart phone has let me do that a couple ways:
First and more importantly it means I'm always connected to my email and voicemail. Being able to respond quickly with a call or email is a big benefit, and any smart phone can do that level of support reasonably well. You didn't ask about that, but I mention it because I feel it is the most important benefit.
Now, back to the phones with SSH. I've used two of them extensively. I had an Cingular 8125 (HTC Windows Mobile) for about 20 months and I now have a Blackberry Curve (8130) which I've used for the last 4 months.
During the 20 months I had a half dozen or so real occasions to use the SSH support. I used it for file transfers, I used it to examine and fix clients machines, and I used it to do other administrative tasks that were only available from my site. That's not often, but for those moments when it was needed it saved me a lot of time and was worth the cost of the data service.
The HTC phone had the better keyboard. On unix systems we use the special characters too much, and getting to those on the BlackBerry is a difficult. Both keyboards are too small to do anything quickly with them. In both cases the screen is really small. The connection is slow (neither unit is 3G), but for a text input it's OK. It reminds me of the old 9600 baud terminals on a main frame. The bottom line is that for occasional access to important information/applications it works just fine. It is in no way useful for extended work. If you a task takes more than 5 minutes you'd be better off finding WiFi at a Starbucks and using your laptop.
My clients are primarily in California and in major cities or along major highways, so that's where I have the most coverage experience. I've found the data service is fine everywhere I go. I use the email, web, etc. very regularly so I tend to notice the loss of data service.
My opinion is that smart phones work OK for occasional emergency SSH access. The keyboard limitations are the weakest link, but given the rest of the constraints on speed, screen, etc. I'm not sure it matters. As long as you're only doing this for short periods in case of a emergency it's a good answer.
A corollary is that if you have something you need to do regularly using the keyboard and SSH on your smart phone, you really ought to set up a web page. The web interfaces on the phones are better and it allows you to control the presentation. It also works better on a wider variety of phones.
