Are you using a Mac as a main computer for bioinformatic work?
I use a few Macs for script and software development. I use the lab's Linux cluster for launching larger analyses.
What positive points do you see about using a mac for bioinformatics?
Unlike most handbuilt Linux desktop options, the hardware and software work well together, so I don't constantly need to be a sys admin like I did with Linux. Wireless networking just works. Plugging in a laptop to do presentations just works. Power management settings just work. The OS X operating system is less fragile to kernel and hardware changes and is easier to fix on my own, when such rare problems arise. OS X can run useful (if still commercial) visualization and productivity tools with easy access to a UNIX terminal, which Linux still cannot do (generally speaking). Many native GUI and command-line tools are available for doing source code control on OS X, so I don't lose any abilities there.
What negative points do you see about using a mac for bioinformatics?
A lot of open source gets distributed as RPM packages or similar, while package systems for OS X are a bit less friendly, in that regard. Some open source GUI apps are X11 based, which not only makes them look ugly but introduces a whole separate set of key shortcuts to learn, just for Linux-y apps. This also makes integration difficult — copying and pasting between native OS X apps is a nice feature, but is difficult between native apps and X11-ized apps.
If you're running a computational cluster, then Linux is a winner here, for the simple fact that the stripped-down hardware you need for that application is cheaper per node. You don't need a Mac Pro's super-duper graphics adapters, for instance, unless you are running custom analyses on fancy GPUs.
On the other hand, now that Thunderbolt shows up as an Ethernet adapter in Mavericks, I'd be interested to see if it gets used as a very fast interconnect for cluster work (a cheaper alternative to Infiniband, for instance).
Ease of installing software (talking about bioinfo software here)?
Depends on the software. Most of it can be compiled easily but you'll need to install Xcode, at a minimum, which used to be a PITA to download (used to need an Apple Developer Center account and navigate Apple's ADC site maze) but it is now pretty easy (OS X App Store + Apple ID + one or two clicks). Just about everything else that you can't compile on your own is something you can manage with macports or homebrew.
On the subject of macports and homebrew, pick one or the other, not both. They will step on each others toes as far as managing dependencies. I prefer macports, but I suspect it is largely a matter of how much you like (or dislike) Ruby, if you want to try homebrew.
Freedom to hack quick solutions for custom problems?
Decently modern versions of Ruby, Python, Perl etc. come pre-installed. So long as your sys admin has given your account admin rights on your Mac, you can compile and install whatever else you need through Apple's Xcode/clang, GNU's gcc and macports or homebrew.
Whatever opinion you want to share about this question
As a political matter, I love Linux's openness and customizability, but as a daily-use operating system, its interface just isn't very well engineered (just one person's opinion) and I suspect that no amount of catch-up work to imitate commercial operating systems will make that aspect of it as easy and reliable to use as OS X. Tools which other staff use for writing and managing papers (Word, Endnote, etc.) just aren't available, and open source and online "equivalents" do not quite make the grade, IMO. Managing document citations in a shared Google Doc is simply a non-starter, for instance. Of course, I'm less amenable to making compromises because I'm a pretty lazy human being. If your time is free and you love the OSS movement and you're willing to put in the work, you can do a lot of these things in Linux.
Why should I (or not) think of getting Macs for our lab?
I suspect it will reduce your administration costs, at the up-front expense of slightly more expensive hardware and the cost of commercial software. You'd need to balance that against the productivity gains from having your staff be able to collaborate together and share data more effectively. Compared with Linux, this is probably a plus mark in favor of OS X and Windows, generally speaking. On the other hand, you're going to spend a bit more up front. Six of one, half dozen of another. You'll need to decide what your priorities are with this investment, I suspect.