It's always a tough question whether you should upgrade to a new version of any software.
Take a look at What's New in Visual Basic 2008 for a detailed summary of the new features in Visual Basic 2008. Here's the extremely high-level overview:
One really, really bad feature is that some of the new tools (such as LINQ) assume you have Option Strict turned off. A very bad idea. You can protect yourself a bit by putting LINQ code in a separate module.
- LINQ - Billed as the greatest thing since sliced bread, I just don't see it. This is a remarkable and powerful feature but I can honestly say in 20+ years of programming I have never really said, "Gosh, I sure wish I could execut SQL-qtyle queries against an array of objects!" It also has some restrictions such as the inability to compose and execute queries at run time. You can weasel around some of this but it's a lot of work. While LINQ isn't my favorite new feature, some of the smaller things Microsoft did to make it happen are worthwhile.
- Extension Methods - Used with restraint, these can be good. For example, you can add encryption or regular expression features to the String class. Don't get carried away, though, or these can lead to confusion.
- Object Initializers - These are slightly useful, making initialization code a bit easier to write and read.
- Relaxed Delegates - This also makes some code a bit easier to write and read.
- If Function - If I had my way, we would go without If and IIf (and GoTo).
- Nullable Types - Nice but not critical.
- IntelliSense Improvements - Mostly these are good, although it does make browsing for things a bit harder at times.
- Improved Framework - A lot of the Framework changes really came with .NET Framework 3.0 a while ago but I'm guessing a lot of people will be seeing them for the first time now. Some of the newer tools such as WPF are extremely interesting. Unfortunately the WPF designers are pretty bad. If you do a lot of WPF programming, you'll probably end up writing a lot of XAML code directly. The results are impressive, though.
So what's the bottom line? There are some good new tools in this release. I don't know whether I would switch in the middle of a large project, although even that might not be too hard. I've upgraded more than 400 programs from Visual Basic 2005 to Visual Basic 2008 and there were only 1 or 2 that needed a few simple tweaks.
I think I would recommend that people start moving to this version as they have time and resources. This is mostly an incremental version not a radical rewrite of Visual Basic so the transition shouldn't be very hard. You'll get the benefits of the new tools when you are ready for them but can ignore most of the differences until then.
At a minimum, you can download Visual Basic 2008 Express Edition and experiment with it for free. For exampe, you can try to upgrade some existing applications and see if they convert easily.