Right now we are in an interesting situation in that you can still buy the original NXT retail set (8527) or order the new NXT 2.0 kit (8547). I have been getting questions about which one to buy, and also what it will mean to have one kit along with project instructions intended for the other kit. LEGO was kind enough to provide me with a pre-release version of the NXT 2.0 kit, which I have been working with for while now, so here is some information, opinions, and recommendations that may help. I will abbreviate NXT 2.0 as NXT2 and the original NXT (8527) as NXT1.
1. First off, I will say that comparing NXT2 to NXT1 when “free building” projects of my own design using only the contents of one kit, I do prefer NXT2. Treads are a welcome addition, the Zamor (small ball) shooter is fun, the color sensor is a very nice upgrade from the light sensor, having two touch sensors is very useful and worth giving up the sound sensor for, and the software has some nice additions. Also, overall I think the mix of building parts is improved, and I find myself generally getting stuck less running out of parts and sizes that I need when limiting myself to one kit.
2. Considering building projects from instructions that you find so far on my site at nxtprograms.com and other web sites and books written for NXT1, unfortunately, most projects intended for NXT1 are not buildable as-is with only the NXT2 kit. The same will also be true for most projects designed for NXT2 if you have NXT1. If you are not limited to just the kit contents and can buy spare parts, this is somewhat less of an issue, but it is something important to be aware of. The kits are very similar overall, but several differences in parts availability and part counts make cross-building difficult. For example, the NXT1 kit has 11 15-beams and 4 13-beams, but NXT2 has 2 15-beams and 10 13-beams. A lot of my NXT1 projects have overall dimensions suited to 15-beams, but for NXT2, 13 is a better maximum dimension. Of course you can get more 15-beams and other parts in various ways, and I have posted some solutions here.
3. The NXT2 software is fully backwards-compatible with NXT1. You can load an NXT1 program (.rbt), and compile and download it to an NXT2 brick. This means that if you buy NXT2, you can use programs intended for NXT1 that you find on nxtprograms.com and other places, or any programs you wrote yourself for NXT1. This also means that if you have a mixture of NXT1 and NXT2, the NXT2 software can handle it all for you (including the sound sensor and other things not directly included in NXT2). However, note that if you connect an NXT1 brick to the NXT2 software and download a program, the NXT2 software will insist on upgrading the firmware of the brick to the NXT2 version.
4. The NXT1 software is not generally compatible with programs written for NXT2. Some very simple NXT2 programs can be loaded into NXT1 and compiled and uploaded to an NXT1 brick, but many cannot. Generally, you will be able to open and view the contents of NXT2 programs saved in the normal .rbt format in order to look at them, but don’t expect to be able to use them. NXT2 also includes a new .rbtx format saved by the new Pack-and-Go feature which is very useful because it includes MyBlocks, graphics, and sound files needed by the program, and is compressed and thus much more compact for all programs, but this format is not readable by NXT1 at all.
Conclusion and Recommendations
If you don’t have any NXT kit yet, and you can manage $360 (plus shipping and tax), the best thing to buy right now in my opinion is NXT2 plus the Education Resource Set (9648) from LEGO Education. NXT2 + 9648 is very close to being a superset of NXT1, so you have (almost) the best of both worlds. You will have NXT2 and everything it can do, you will have enough parts to build most models intended for NXT1 (from web/books/etc), and you will have a number of other cool misc. special parts as well. You will still be missing some NXT1 parts. Here is an analysis of the missing parts and suggestions for where to get them if necessary.
If you are new to NXT, want to spend less than $300, and want a set primarily to design and build your own projects, get NXT2.
Teachers and others that are dependent on existing instructions for NXT1 may want to stick with NXT1 (or the Education version of the NXT) for a while, then start saving money and putting together a plan for how to move to NXT2 later. Note that all of your NXT1 equipment will still be usable (bricks, motors, sensors, spare parts) when you transition/upgrade.
If you are new to NXT, have a budget of only $250 or so, want to be able to primarily build projects from existing instructions available on the web and books, and can’t wait, NXT1 is still available and is still a great kit. Then if and when you get an NXT2 set later, you can upgrade the firmware on your old brick and switch to the NXT2 software, and then you will have a great two-kit combination that will allow lots of interesting projects, including NXT-to-NXT Bluetooth projects.
For fans of existing project instructions, though, please also consider that LEGO has said that the mindstorms.com web site will be posting (free) instructions for several NXT2 projects in addition to the four that come with the NXT2 set. I will also be posting NXT2 projects to nxtprograms.com when and as time allows, and many other NXT2 users will start to post their projects to other places such as NXTLOG, and the variety will grow over time!