While I'm not a patent agent, I've been working with IP (primarily patents) for (oddly enough) about 17 years.
A patent agent typically works with patent prosecution -- writing and rewriting a patent, responding to the examiner, etc., to get the patent granted.
Most of what I've done is with patents that have already been granted, and trying to figure out whether the patent applies to a particular product.
A few points about being a patent agent:
At least from what I've seen, pressure is rather higher than in a typical software development job. As a patent agent, virtually everything you do is working to a hard deadline -- when a patent examiner sends a notice of a decision, you must respond within the specified period of time, or they will assume the application has been abandoned. In the case of a particularly important patent, being even one day late could lead to huge losses.
If you aren't an avid language lawyer, you may not have the personality for the job. Although they're only allowed to work on patent-related work, what you're calling a "patent agent", the US PTO calls a "patent attorney".
The test to become a patent agent is 2 days long, and excruciatingly difficult. Just for example, Gary Boone is an electrical engineer who invented the microcontroller while he was at TI. He's since become a patent agent, and says the test to become a patent agent was the hardest test he ever took (and he's definitely a smart guy, to put it mildly). Plan on being able to quote chapter and verse of both the applicable law and the Manual of Patent Examining Procedure on virtually any relevant point before you can hope to pass the test.
Looking at the work I do/have done, dealing with patent enforcement:
It doesn't have the constant pressure of patent prosecution. Most of the time, the pressure level is about like a typical software developer's job. Then, during a court case, the pressure can simply go off the charts. Essentially the smallest patent case anybody (reasonable at all) is likely to contemplate is going to be in the $10 million range, and I've personally been involved in a couple that were in the $1 billion (milliard, for those outside the US) range. I don't have numbers for all of them at hand, but I'd guess that in the cases I've worked on, the median would come out to something like $250-300 million.
Work hours can go along with that. Normally, about like a typical programmer (somewhere around 50 hours/week). Especially toward the end of a big case, that can go up a lot though. I've worked a few 100+ hour weeks, and seen others do even more (sometimes going as far as sleeping at a hotel across the street from the office to avoid the half hour drive each way to sleep at home).
The work is usually interesting though. Contrary to popular belief, most patents honestly do have something pretty cool and interesting -- and sometimes something that's truly brilliant. Working with them all the time, you frequently get to see/read about some really cool ideas (and, of course, some harebrained nonsense as well).
Aside from reading patents, I also spend quite a bit of time designing and running tests to figure out how a specific part of a system works, to see whether or not it does what the patent claims cover. Figuring out a way to prove one way or another can be (and often is) quite a bit of fun. When I write software for this (which is pretty frequently) I get to concentrate almost exclusively on the functionality -- just for example, it's generally quite acceptable for it to look quite utilitarian, as long as it does what it's supposed to.
You probably do need to be prepared to spend a fair amount of time outside work keeping up on what's happening with technology in general. Most work will deal with the most minute details of something extremely specific, but the specific thing you're looking at will change from one task to the next. I've gone (for example) from dealing with automated tools for database normalization one day, to an improved method of laying out traces in an IC to prevent electro-migration the next (though, in fairness, I probably work on a wider variety of projects than most people -- a fair number might go for years at a time without ever stepping outside the bounds of, say, mobile phone protocols).
