Amid the S&P 500's nearly 5% gain over the past six months, one subsector has done exceptionally well: software-as-a-service (SaaS) stocks, particularly those providing platforms and applications to help businesses. The market seems head over heels for these stocks. Cloud-based customer relationship management company salesforce.com (NYSE: CRM), financial software company Intuit (NASDAQ: INTU), e-commerce platform Shopify (NYSE: SHOP), and payment processor Square (NYSE: SQ) have all soared 30% or more during the last six months alone.Is the market's bullishness toward these four companies justified, or has optimism for these SaaS stocks gotten carried away?