So Matty gave some reasons why people might consider switching from Solaris to Linux.
This is all a matter of opinion, of course. Despite computers being binary systems, everything in IT is shades of grey (often very dirty shades of grey). So here's my take on Matty's points, in the same order:
1. Integrated LAMP stack. Actually, all my Solaris boxes could have a full SAMP stack installed, but I would never use it. I wouldn't (and didn't) use it on Linux either. In both cases I would much rather install the application stack separately. It's very much safer that way - not only can I be sure that all the components are at specified known levels, and are the same across platforms, but I can be sure that my OS vendor isn't going to screw me around. I always had to wipe all evidence of apache, mysql, and friends from a RedHat system in order to get operational stability; unfortunately it appears that Sun are heading down the same misguided path by bundling more services in Solaris.
2. At my previous place of employment, you could always tell when the next version of Fedora had been released - all the group of developers were surly and miserable for a week because there desktop had been randomly rearranged and they had to relearn it. I reckon each of them lost a week's productivity as a result. Sun aren't much better - open source software doesn't care for operational stability, and every time Sun update Gnome or JDS in Solaris nothing works. And I'm now convinced we're going backwards. (As an aside, I've had many more problems with desktop apps not working right on Fedora than Solaris, but that may have been because the Solaris versions didn't try to push the bleeding edge so hard.)
3. The JES stack deserves a good kicking. My experience of it has been woeful. So I agree with that part. The other side of the coin is regular updates. As I said in point 1, I don't want my OS provider to randomly modify the working of key applications. So I wouldn't use either of them.
4. My experience of ISVs is that they hate supporting Linux. And I can't blame them - the qualification effort is horrendous. I asked one the other week about Solaris x86 support, and their answer was "we keep getting asked about that, but it's not on our roadmap". Anothe one went "phew!" when we said we were going to deploy on Solaris - he was then confident it would work out of the box. But, yes, there does seem to be more visibility for Linux - I suspect vendors think that's what customers want, when most of the time any alignment being customer requirements and vendor offerings is random at best. At least with Solaris, if it works it works, rather than having to stick to one particular release of one particular Linux distribution with a specific set of patches.
5. Not so; managing applications and patches on Solaris is an absolute doddle. Provided you ignore completely Sun's perpetually failed attempts to improve the process - patchpro, smpatch, prodreg, update connection are all worthy only of the dustbin. Fortunately it's trivial to write your own tools, or use pkg-get or pca which are massively better than anything Sun have come up with so far.
6. Why not upgrade? Modulo bugs in occasional releases (and these are the sorts of bugs that mean that not every build qualifies as a regular Express release) regular upgrade or live upgrade work fine. There's no need to futz with bfu unless you really want to.
7. At least with Solaris you have the option of zones! Are zones a universal panacea? No. But they are enormously useful for a whole range of operations where you need to consolidate or isolate services. I can believe smpatch update could take days, but then I wouldn't do it like that - I wouldn't use smpatch, for starters, and I wouldn't create the zones and then apply the patches. For this sort of thing our mode of operation was to simply migrate services onto a zone on a different box, and then rebuild machines, if the patch overhead was too great. (Based on the philosophy of never patching or rebooting a live service.) The fact is that there's no getting away from the fact that by creating 25 zones, you've increased the update cost 25 times. It is certainly true, though, that there's room for improvement in the patch tools. They are significantly slower than can be accounted for by the actual work they're doing.
8. It's a tricky balancing act. And I thin Sun have done pretty well with OpenSolaris. There was massive concern that the open source process would destroy Solaris' core strengths, and it doesn't seem to have done so yet. My hope was that it would address some of Solaris' weaknesses, and I don't think it's actually done that yet, but the level of engagement is there, and the signs are promising. To be honest, I'm not sure I would want to use an OS that would allow outsiders to put code back to the kernel source tree - does Red Hat allow me to modify their kernel source?
One thing that worries me about end-user putbacks is that the whole thing has to be vetted and managed, and that has to be done by Sun (because their position is that if it goes into OpenSolaris then it goes into Solaris - there is only one codebase), so it costs Sun more to have a community member fix something than to have one of their own engineers do it. Given the financials of the past few years, this doesn't strike me as an optimal situation.
There are some good points here - it's not as if Solaris hasn't got any weaknesses. The real killer is the availability of commercial software. If you're only offered it under Linux then you're going to have to put up with increased costs in terms of licensing, support, hardware, and staffing (or just pretend they don't exist), or choose another product. But there are many places where one person's ideal is anathema to someone else, and we just have to accept that there is no one solution to every problem.