Overview

The Cayman and Boxster share the same architecture, and transaxles in them are pretty much identical.  

These are not as strong as the G50s (in fact, the 5 speed versions are really versions of the Audi 01E transaxles), but are available, and are inexpensive.

Interestingly, these are also available as remanufactured units for relatively low prices, making them- if your engine is not making too much power- an interesting choice.

Sources

These transaxles are widely available from local boneyards, specialty Porsche-specific wrecking yards, several different re-builder shops including Gbox, California Motorsports, and others.

They can also be purchased new, or re-manufactured, in some cases, directly from Porsche or one their parts resellers.  For example, new/remanafactured boxes from the Boxster/Cayman series are available from Porsche as well as others. The transaxles generally have a wide aftermarket that can provide solutions to known problems.

Technical

The 5-speeds are from the non-S cars, while the S cars had the 6-speed transaxle.  The 6-speeds are generally thought to be better suited to a V8, as they are typically stronger, have taller gears, and don't cost that much more as new or used units.

One key advantage of the Boxster/Cayman transaxles is that they don't need to be inverted- they were designed from the beginning to be run as a mid-engined transaxle.  That saves some small effort compared to the G-series transaxles from the 911-series cars.

Starters, clutches and flywheels are available to make these units work with the popular LS V8, but other engines will need custom adapters, and possibly flywheels, etc.

Superlite has adapters for this series of transaxle.

Issues

The 5-speeds are not as strong as the 6-speed units.  The torque limits are lower on these transaxles than most of the G-series transaxles, and unlike the G-series boxes, there are very few if any different choices for gears.  What you have is pretty much what you get.

As with any transaxle, they will break when abused with high torque and sticky tires, or when used to make drag-style starts.  Some have reported that even with high power, these units can be made to last if they are not speed shifted, or shock-loaded with hard starts.

The gearing from the factory is designed for a relatively low-torque 6-cylinder engine, not a V8.  Consequently, you may have to live with a buzzier rev range at cruise on the highway.  That, and the lower acceptable torque rating for thiese units are why these aren't as popular as the G-series units, despite the significant price difference.

Until recently, the tooling and expertise to rebuild these transaxles was not widely available.  Even now, some shops insist they cannot be rebuilt or repaired.  As such, the shop you choose may not have any real experience fixing or rebuilding them, so be very specific if this is important to you.  That may be why the new or reman prices are so affordable, as in the past, any damage to the transaxle always resulted in the purchase of a new unit, increasing the volume, with the possibility that the increased volume made pricing better over time.

 Ancillaries

Many different starters are available, both Porsche and aftermarket. Clutches, and flywheels are also easy to source, and can be targeted to many different levels of power